.. index::
   single: Excited states
   single: Vertical spectra
   single: Rydberg states
   single: CASPT2

.. _TUT\:sec\:excited:

Excited states
==============

.. only:: html

  .. contents::
     :local:
     :backlinks: none

The accurate calculation of excited electronic states has been
a challenge for quantum chemistry. The possibility for
accurate calculations of such states in molecules has only
recently been made possible through the development of new
quantum chemical techniques. CASPT2 is currently one of the more
successful methods to compute excited states due to its balance
between accuracy and cost. In addition to the intrinsic
limitations of the method, photochemistry and photophysics
involves a large number of situations and mechanisms which
complicate the problems enormously. In the present section we
are going to show a systematic way to deal with a large
number of states in a molecule. We have selected the thiophene
molecule and our goal will be to compute the lowest valence and
Rydberg singlet states at the ground state geometry. This can
be considered to be the gas-phase absorption spectrum of the molecule.
The calculations comprise an extensive use of the RASSCF,
CASPT2, and RASSI programs. Selection of proper active spaces,
building of appropriate diffuse basis functions, calculation
of transition dipole moments, and use of the level-shift technique
in CASPT2 will be some of the topics covered.

.. index::
   single: Thiophene

.. _TUT\:sec\:thiophene:

The vertical spectrum of thiophene
----------------------------------

Besides the usual limitation typical of any *ab initio* procedure
due to the size of the system and the calculation of the integrals,
the CASPT2 method has the basic limitation of the size and selection of the
active space in the preliminary CASSCF step, not only because the
space cannot be too large but because the active space defines the
type and number of configurations (read excitations) to be included
in the multiconfigurational wave functions.
The near-degenerate configurations describing all states must
be present in the reference wave function.
Therefore, certain knowledge of the system is necessary
to design the calculation and, for excited states, this will
limit the number of states we are able to study.

Planning the calculations
.........................

Thiophene is a planar five membered ring molecule containing one sulfur
and four carbon atoms. The :math:`\pi` structure of the system contains
two conjugated double bonds between carbon atoms. Therefore, the orbital
:math:`\pi` valence structure is composed by two :math:`\pi` bonding, two
:math:`\pi^*` antibonding orbitals, and one :math:`\pi` nonbonding orbital placed
on the sulfur atom.
The :math:`\pi` orbitals are the highest occupied ones in this type of
systems and excitations from them form the UV
spectrum in gas phase and solution. Also, typical orbitals involved
in low-lying excited states are the lone-pair orbitals such as the
sulfur :math:`n` orbital co-planar with the :math:`\sigma` skeleton of the
molecule. On the other hand, :math:`\sigma` orbitals forming :math:`\ce{C-H}` and :math:`\ce{C-C}`
bonds do not participate in the low-lying excited
electronic states. One has, however to be careful here. In thiophene there are
low-lying virtual :math:`\sigma` that give rise to excited states in the region around
6 eV :cite:`Tozer:99a`.

.. figure:: thiophene.*
   :name: fig:thiophene
   :width: 50%
   :align: center

   Thiophene

.. index::
   single: Active space

With this in mind we have to include at least the three :math:`\pi`
and two :math:`\pi^*` valence orbitals and the valence :math:`\sigma` lone-pair
on the sulfur in the active space. The molecule belongs
to the |Ctv| point group, therefore we have three |bo| and
two |at| :math:`\pi`,\ :math:`\pi^*` orbitals and one |ao| :math:`n` orbital.
That is, our minimal valence active space can be labeled
(1302), where each number corresponds to the number of |ao|, |bo|,
|bt|, and |at| orbitals, respectively.

.. index::
   single: Orbitals; Rydberg functions
   single: Orbitals; Rydberg
   single: Rydberg states

But the valence states are not the only states present at low energies.
In a gas-phase spectrum of a neutral molecule the Rydberg states start to
appear at energies above 5 eV. Therefore, they must be simultaneously
included in the calculations. The Rydberg orbitals are large compared
to the molecular dimension and therefore have quasi atomic shapes. Rydberg
states are commonly labeled as excited states of atoms with a principal
quantum number :math:`n` and the usual angular quantum numbers :math:`l` and :math:`m`.
For molecules containing only first row atoms :math:`n` conventionally starts
with 3. This convention is actually used also in a molecule like thiophene,
although in sulfur the valence electrons are in the third shell.
Increasing the value of :math:`n` will lead to more and more diffuse orbitals,
eventually converging to an ionized state of the molecule. The lowest
Rydberg state corresponds to the excitation HOMO\ |->|\3\ |s|.
The next components will be 3\ |px|, 3\ |py|, and 3\ |pz|, followed by the
five components of 3\ |d.|.

The Rydberg orbitals classify into the point group like their corresponding
atomic orbitals. Therefore, a look at the character table
(see :numref:`tab:c2v`) indicates that in |Ctv| the |s|, |pz|, |dzt|,
and |dxtyt| Rydberg orbitals belong to symmetry |ao|, |px| and |dxz| to symmetry
|bo|, |py| and |dyz| to symmetry |bt| and, finally, |dxy| to
symmetry |at|. According to the labeling defined above the nine lowest Rydberg
orbitals classify to (4221). It is obvious that we cannot normally
afford to have simultaneously the whole valence plus Rydberg space
(15 active orbitals in the present example). Therefore we are going to
exploit the symmetry properties to select different active spaces.

.. index::
   single: Orbital energies

By inspection of the SCF orbital energies or the ionization
potentials of the molecule we observe that the highest occupied
orbitals HOMO (1\ |at|) and HOMO\ |-|\1 (2\ |bo|) are reasonably close in
energy (around 0.6 eV). Therefore, two Rydberg series close in energy
can be expected at low energies, the first one arising from the
HOMO orbital and the second from the HOMO\ |-|\1 orbital. By exciting
one electron from each of those orbitals to each one of the
Rydberg orbitals we know the symmetry of the resulting state.
For instance, the excitation HOMO (|at|) |->| 3\ |s| (|ao|) leads
to a :math:`A_2` by direct product of the symmetry representations.
:numref:`tab:thio` contains the analysis for the Rydberg states
arising both from HOMO and HOMO\ |-|\1 orbitals to the :math:`n=3` Rydberg
orbitals. They form the two lowest Rydberg series. We want also
to locate the state from the lone-pair HOMO\ |-|\2 (11\ |ao|) to 3\ |s|.

.. float::
   :type: table
   :name: tab:thio
   :caption-top:
   :caption: Selection of active spaces in thiophene.

   .. _tab\:thio_a:

   =================== ==== ==== ==== ====
   |zws|               Symmetries
   =================== ===================
   \                   |ao| |bo| |bt| |at|
   Frozen orb.         5    1    3    0
   Inactive orb.       6    0    4    0
   Valence active orb. 1    3    0    2
   =================== ==== ==== ==== ====

   .. |pa2| replace:: (:math:`\pi`) |at|\ |->|
   .. |pb1| replace:: (:math:`\pi`) |bo|\ |->|
   .. |na1| replace:: (:math:`n`) |ao|\ |->|
   .. |HOMO0| replace:: HOMO\ |->|\ :math:`n=3`
   .. |HOMO1| replace:: HOMO\ |-|\1\ |->|\ :math:`n=3`
   .. |HOMO2| replace:: HOMO\ |-|\2\ |->|\ :math:`n=3`
   .. |A1| replace:: :math:`A_1`
   .. |A2| replace:: :math:`A_2`
   .. |B1| replace:: :math:`B_1`
   .. |B2| replace:: :math:`B_2`

   .. _tab\:thio_b:

   ============ ============ ====== ============ ============ ====== ============ ============ ======
   Rydberg states
   --------------------------------------------------------------------------------------------------
   |HOMO0|                   State  |HOMO1|                   State  |HOMO2|                   State\
                                                                                               [#a]_
   ========================= ====== ========================= ====== ========================= ======
   |pa2|        3\ |s| |ao|  |A2|   |pb1|        3\ |s| |ao|  |B1|   |na1|        3\ |s| |ao|  |A1|
   \            3\ |p.| |ao| |A2|                3\ |p.| |ao| |B1|
   \            3\ |p.| |bo| |B2|                3\ |p.| |bo| |A1|
   \            3\ |p.| |bt| |B1|                3\ |p.| |bt| |A2|
   \            3\ |d.| |ao| |A2|                3\ |d.| |ao| |B1|
   \            3\ |d.| |ao| |A2|                3\ |d.| |ao| |B1|
   \            3\ |d.| |bo| |B2|                3\ |d.| |bo| |A1|
   \            3\ |d.| |bt| |B1|                3\ |d.| |bt| |A2|
   \            3\ |d.| |at| |A1|                3\ |d.| |at| |B2|
   ============ ============ ====== ============ ============ ====== ============ ============ ======

   .. _tab\:thio_c:

   +---------------------------------------------------------------------------------------------------+
   | Total active space                                                                                |
   +========================================================+==========================================+
   | | |A1|, |B2| states (:math:`\pi\to\pi^*`)              |                                          |
   | | |A1|, |B2| states (:math:`\pi\to\mathrm{R}(\pi^*)`)  | Valence (1302) + Rydberg (0201) = (1503) |
   | | |A2|, |B1| states (:math:`n\to\pi^*`)                |                                          |
   +--------------------------------------------------------+------------------------------------------+
   | | |A2|, |B1| states (:math:`\pi\to\mathrm{R}(\sigma)`) | Valence (1302) + Rydberg (4020) = (5322) |
   | | |A1| states (:math:`n\to\mathrm{R}(\sigma)`)         |                                          |
   +--------------------------------------------------------+------------------------------------------+

   .. [#a] Only considered up to the :math:`A_1` (3\ |s|) state because the remaining are expected at higher energy.

.. index::
   single: Active space

The computed states will use different partitionings of the active space. The
basic valence space (1302) must be included in all the cases. The valence
:math:`\pi\to\pi^*` states only involve excitations into the :math:`\pi` and :math:`\pi^*`
orbitals. Therefore they belong to the :math:`A_1` and :math:`B_2` symmetries. In addition
we can have single excitations (Rydberg states) from the occupied :math:`\pi`
orbitals to the Rydberg orbitals of :math:`b_1` and :math:`a_2` symmetries. The number of
Rydberg orbitals belonging to those symmetries is (0201). Thus, the final space
to compute simultaneously valence and Rydberg :math:`\pi\to\pi^*` states is
(1302) + (0201): (1503). The same space can be used to compute
:math:`n\to\pi^*` states because the :math:`n` orbital and the :math:`\pi^*` orbitals
are included into the active space. The symmetries of these states, however,
will be :math:`A_2` and :math:`B_1`. In the table we also have another
division for the :math:`A_2` and :math:`B_1`, :math:`\pi\to\mathrm{R}(\sigma)`, and :math:`A_1`, :math:`n\to\mathrm{R}(\sigma)`,
(only the :math:`n`\ |->|\3\ |s|) Rydberg states, using an active space (5322).
We have, therefore, divided the excited states to be computed by symmetries
and active space. State-average CASSCF calculations for each one of
the cases have to be performed. The only question which remains is how many roots
we have to include in each of the cases. This is also determined by the symmetry
and active space available. For instance, for the :math:`\pi\to\pi^*` :math:`A_1` states,
we want to compute the ground state plus three Rydberg states (see :numref:`tab:thio` in both
HOMO and HOMO\ |-|\1 |->| :math:`n=3` series) plus a certain number of valence states.
If we do not have any previous experience we may think of three or four possible
valence states but we know that the usual number of low-lying valence
states is close to the number of valence singly excited states, in this case
two of :math:`A_1` symmetry. This does not mean that the states are
going to be described by one single configuration; it is simply an estimation
of the number of relevant states based on experience. In summary, we expect
to compute six :math:`A_1` states and therefore we include six roots in the
CASSCF state-average input.

It is not uncommon that one or more valence states do not appear in the
initial CASSCF calculation including the desired roots and other higher Rydberg
states. This is
due to the fact that valence states usually require larger dynamical
correlation corrections than the Rydberg states. Therefore in a CASSCF
calculation the Rydberg states are, in general, lower in energy than the valence states.
The dynamical correlation included by the CASPT2 method will place the
states correctly. However this is only possible if the states are present
in the CASSCF calculation. It is then necessary to be sure that the states
are located at the CASSCF level. Maybe it is necessary to increase
the number of roots and in special cases like those with low symmetry
even to delete Rydberg orbitals from the active space
:cite:`Roos:95a,Roos:96b,Serrano:96a,Serrano:96b`.

In the following we will describe briefly the calculations :cite:`Serrano:94th`.
A detailed report of the vertical excited spectrum of thiophene can be found
in references :cite:`Serrano:94th,Serrano:93d`. The selection of the active spaces in that
work included additional orbitals to minimize the effect of intruder
states. The availability of the level-shift technique in later versions of
|molcas| allow us to use a smaller active space.

.. index::
   single: Rydberg orbitals
   single: Orbitals; Rydberg
   single: Basis set; Diffuse functions
   single: Basis set; Rydberg functions

.. _TUT\:sec\:make_rydberg_basis_sets:

Generating Rydberg basis functions
..................................

First we describe a method for generating Rydberg basis functions
for molecules.
Such Rydberg orbitals are diffuse and thus require
diffuse basis functions. Due to this diffuseness they are not
"localized" to atoms in the sense that valence orbitals are, but
should be considered to be spread out over the entire molecule.

The basis of the method lies in the fact
that if we add an electron into a virtual orbital, the energy
for the system is increased by the orbital energy, according to
Koopmans' theorem.
The reorganizational effects are very minor for the diffuse
virtual orbitals. Thus adding an electron into a virtual orbital
for a cation is an reasonable approximation to the proper
Rydberg state.
A more extensive discussion of the method
outlined below can be found in :cite:`Roos:96b`.

The method can be broken down into a few steps (see Ref. :cite:`Roos:96b` for details):

#. Perform a RHF or valence CASSCF calculation of the system with one electron
   removed, using the :program:`RASSCF` program.
   This will determine the center of charge which is
   a suitable choice to center the Rydberg basis function
   expansion. The result is rather insensitive to this choice.

#. Add a suitable diffuse primitive basis set at the center of charge.
   We use as universal exponents those optimized by Kaufmann *et al.* :cite:`Kaufmann:89`
   for Rydberg wave functions.

#. Repeat the RHF or CASSCF calculation in the new basis.

#. Construct the basis set using the program :program:`GENANO` and use the lowest virtual
   function to define the basis set.

It is better not to use an extremely large valence basis set to
perform these calculations. The best choice is a double-zeta or
double-zeta plus polarization basis set.
In this example we will use benzene which have a natural
origin in the center of the ring.
Thus we have eliminated the step of determining the
center of charge.
Also we have made the simplification of only considering s-functions.

The procedure we will follow is

#. Create inputs for :program:`SEWARD`, :program:`SCF`, :program:`RASSCF`,
   and :program:`GENANO`.

#. Create a shell script to run
   :program:`SEWARD`, :program:`SCF`, and :program:`RASSCF`,
   and run the job.

#. Hand edit the resulting formated orbital file, :file:`C6H6.RasOrb`.
   Set the occupation numbers for the occupied space to zero, while
   the first three virtual orbitals in the first irreducible representation
   get the occupation numbers :math:`10^{-1}`, :math:`10^{-2}` and :math:`10^{-3}`
   respectively. These occupation numbers are quite arbitrary as long
   as they form a decreasing sequence.

#. Create a shell script to run :program:`GENANO` and run the job.

#. The resulting file :file:`C6H6.Ano` now contains the contraction coefficients.
   Merge this file with the exponents in the :program:`SEWARD` input to obtain the final
   contracted basis set. We normally use only one function of each type.

The radial extent of the resulting basis functions is shown in
:numref:`fig:rydberg_orbitals`.

.. figure:: ex-99.*
   :name: fig:rydberg_orbitals
   :width: 75%
   :align: center

   Radial extent of the Rydberg orbitals.

Here are the inputs used for this example. First the SEWARD input
using the uncontracted Rydberg functions (note that only the s-type Rydberg
basis is shown).

.. extractfile:: advanced/SEWARD.Rydberg.input

  &SEWARD &END
  Title
   Benzene molecule.
  Symmetry
  X Y Z
  *OneOnly
  Basis set
  C.ano-s...3s2p1d.
  C1    2.636169     .000000     .000000
  C2    1.318084    2.282990     .000000
  End of basis
  Basis set
  H.ano-s...2s1p.
  H1    4.684633     .000000     .000000
  H2    2.342316    4.057011     .000000
  End of basis
  Basis set
  X....8s8p8d. / Inline
    0.0 0
  8 8
  .02462393 .01125334 .00585838 .00334597 .00204842 .00132364 .00089310 .00062431
  1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
  0.0 1.0 0.0 0.0 0.0 0.0 0.0 0.0
  0.0 0.0 1.0 0.0 0.0 0.0 0.0 0.0
  0.0 0.0 0.0 1.0 0.0 0.0 0.0 0.0
  0.0 0.0 0.0 0.0 1.0 0.0 0.0 0.0
  0.0 0.0 0.0 0.0 0.0 1.0 0.0 0.0
  0.0 0.0 0.0 0.0 0.0 0.0 1.0 0.0
  0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.0
  X     0.000000    0.000000     .000000
  End of basis
  End of input

Once computed, the contracted functions will replace the uncontracted ones.
In the usual calculations we are going to use one function of each type,
1s1p1d, but we can keep three of them if we want to increase the Rydberg
basis for some particular use. Here is the input listing for the generation of
the ANO. Note that in newer versions of |molcas| the sequence of calculations is
driven by the input list. You can skip parts of the calculation by commenting
out (with a ``*``) the corresponding namelist input (for example ``* &SEWARD &END``
skips the integral calculation).

.. extractfile:: advanced/GENANO.C6H6.sample

  &SEWARD &END
  Title
   Benzene molecule.
  Symmetry
  X Y Z
  *OneOnly
  Basis set
  C.ano-s...3s2p1d.
  C1    2.636169     .000000     .000000
  C2    1.318084    2.282990     .000000
  End of basis
  Basis set
  H.ano-s...2s1p.
  H1    4.684633     .000000     .000000
  H2    2.342316    4.057011     .000000
  End of basis
  Basis set
  X....1s1p1d. / Inline
    0.0 0
  8 1
  .02462393 .01125334 .00585838 .00334597 .00204842 .00132364 .00089310 .00062431
     .15531366  -.26126804   .38654527
   -1.53362747 -1.27182240   .94560891
    1.10186802   .95250581 -1.24269525
   -1.70918216   .49632170 -2.22724281
    2.03031830   .68292933  1.94719179
   -1.73187442  -.56245782   .68883478
     .92694465   .30675927   .15138171
    -.22934028  -.07852136  -.02092438
  X     0.000000    0.000000     .000000
  End of basis

  &SCF &END
  Title
   Benzene molecule.
  Occupied
   6 5 4 3 1 1 1 0
  End of input

  &RASSCF &END
  Title
   Benzene molecule
  Symmetry
   7
  Spin
   2
  nActEl
    1 0 0
  Inactive
   6 5 4 3 1 1 0 0
  Ras2
   0 0 0 0 0 0 1 0
  LumOrb
  Thrshld
  0.5d-8 0.5d-4 1.0d-4
  Iterations
   50 25
  End of input
  >>COPY $Project.RasOrb  NAT001
  >>COPY $Project.OneInt  ONE001
  >>COPY $Project.RunFile RUN001

  &GENANO &END
  Title
   Rydberg basis set for benzene.
  sets
   1
  Center
  X
  Weights
   1.0
  end of input

.. index::
   single: Program; GENANO
   single: GENANO

Here is the shell script used for this example. It is written in Korn shell,
but no exotic features of Korn shell are used, so rewriting them into
C shell, or whatever your favorite shell is, is a straightforward matter.

.. index::
   single: Shell script

::

  #!/bin/ksh
  Project='C6H6'
  Home=$PWD
  WorkDir=/temp1/$LOGNAME/$Project
  export Project WorkDir
  print 'Start of job:' $Project
  print 'Current directory:' $Home
  print 'Scratch directory:' $WorkDir
  #
  trap 'exit' ERR
  rm -fr $WorkDir
  #
  molcas  $Home/$Project.input >$Project.output
  #
  rm -r $WorkDir

For thiophene one can proceed in the same way. The only
difference (apart from the fact that we generate s,p,d
functions) is that two states of the cation are going to
be computed and therefore the final step using the
GENANO program will involve two files and have the following input: ::

  !ln -s $Home/Thiophene.Ano      ANO
  !ln -s $Home/Thiophene.RasOrb1  NAT001
  !ln -s $Home/Thiophene.RasOrb2  NAT002
  !ln -s Thiophene.OneInt         ONE001
  !ln -s Thiophene.OneInt         ONE002

  &GENANO &END
  Title
   Rydberg basis set for thiophene.
  sets
   2
  Center
  X
  Weights
   0.5 0.5
  End of input

The charge centroid is chosen as an average of the charge centroids
of the two cations.

.. index::
   single: Program; Seward
   single: Seward
   single: Program; SCF
   single: SCF
   single: Program; RASSCF
   single: RASSCF
   single: Basis set; For excited states

SEWARD and CASSCF calculations
..............................

Once we have built the diffuse basis set we can proceed with the
SEWARD and CASSCF calculations of the different states. Remember that
no quantitative result can be expected for calculations which use
less than a DZP basis set. Additionally, as we are using methods
which include large amounts of correlation, it is also recommended
to use basis sets designed to include the correlation, such as the
Dunning correlation-consistent basis sets or the Atomic Natural
Orbital-type basis sets. Several tests of the accuracy of the
ANO-type basis sets for calculations on excited states can
be found elsewhere :cite:`Fuelscher:94a`. It was found that the
minimum basis set suitable for calculations on excited states
is the ANO 3s2p1d basis set for the first row atoms, with
2s functions for the hydrogen. The recommended basis however is
an ANO 4s3p1d basis set.

We proceed with the calculations on thiophene. The inputs for
the programs :program:`SEWARD`, :program:`SCF`, and :program:`RASSCF`
(:math:`^1A_1` states) are: ::

  &SEWARD &END
  Title
  Thiophene molecule. Experimental gas-phase geometry.
  Symmetry
   X Y
  Basis set
  S.ANO-L...5s4p2d.
  S1    0.000000  0.000000  0.000000  Bohr
  End of basis
  Basis set
  C.ANO-L...4s3p1d.
  C1    0.000000  2.333062  2.246725  Bohr
  C2    0.000000  1.344416  4.639431  Bohr
  End of basis
  Basis set
  H.ANO-L...2s1p.
  H1    0.000000  4.288992  1.677364  Bohr
  H2    0.000000  2.494694  6.327573  Bohr
  End of basis
  Basis set
  X....1s1p1d / Inline
   0.0000000 2
  *  s-type diffuse functions
      8    1
   .024624 .011253 .005858 .003346 .002048 .001324 .000893 .000624
    .38826283
  -1.91720062
   1.70115553
  -2.69265935
   3.15654806
  -2.69329518
   1.44320084
   -.35712479
  *  p-type diffuse functions
      8    1
   .042335 .019254 .009988 .005689 .003476 .002242 .001511 .001055
    .14713386
   -.64370136
   -.17112583
   -.62433766
    .58193247
   -.53426167
    .30777301
   -.08250038
  *  d-type diffuse functions
      8    1
   .060540 .027446 .014204 .008077 .004927 .003175 .002137 .001491
    .24501363
    .04635428
    .66592833
   -.08963981
    .52211247
   -.32807746
    .18219220
   -.04616325
  X               .0000000000         .0000000000         .1609268500
  End of Basis
  End of Input

  &SCF &END
  Title
   Thiophene molecule
  Occupied
  11 1 7 3
  Iterations
  40
  End of Input

  &RASSCF &END
  Title
   Thiophene. pipi  1A1 states
  Symmetry
      1
  Spin
      1
  Nactel
      8    0    0
  Frozen
      4    1    3    0
  Inactive
      6    0    4    0
  Ras2
      1    5    0    3
  CiRoot
  6 6
  1 2 3 4 5 6
  1 1 1 1 1 1
  Iter
  50,25
  LumOrb
  End of Input
  >> COPY $Project.JobIph $CurrDir/$Project.1A1.JobIph

The last line will copy the current JOBIPH file to a file in the directory where
the job was submitted.

The wave function and natural occupation numbers obtained for the
:math:`^1A_1` states are:

.. index::
   single: RASSCF; CI coefficients
   single: RASSCF; Configurations
   single: RASSCF; Natural occupation
   single: Orbitals; Natural

::

                                    Wave function printout:
  occupation of active orbitals, and spin coupling of open shells (u,d: Spin up or down)

        printout of CI-coefficients larger than  0.38 for root  1
        energy=   -551.412548
        conf/sym  1 22222 444     Coeff  Weight
              11  2 22000 200   0.95720 0.91624

        printout of CI-coefficients larger than  0.38 for root  2
        energy= -551.192455
        conf/sym  1 22222 444     Coeff  Weight
              14  2 22000 u0d   0.38522 0.14839
              20  2 2ud00 200   0.68777 0.47302

        printout of CI-coefficients larger than  0.38 for root  3
        energy= -551.178212
        conf/sym  1 22222 444     Coeff  Weight
              85  2 2u0d0 200   0.74016 0.54783
              86  2 2u00d 200   0.46282 0.21421

        printout of CI-coefficients larger than  0.38 for root  4
        energy= -551.155996
        conf/sym  1 22222 444     Coeff  Weight
              12  2 22000 ud0   0.49009 0.24019
              14  2 22000 u0d   0.72977 0.53257

        printout of CI-coefficients larger than  0.38 for root  5
        energy= -551.151801
        conf/sym  1 22222 444     Coeff  Weight
              85  2 2u0d0 200  -0.48463 0.23486
              86  2 2u00d 200   0.78218 0.61180

        printout of CI-coefficients larger than  0.38 for root  6
        energy= -551.106218
        conf/sym  1 22222 444     Coeff  Weight
               1  2 22200 000  -0.50027 0.25027
              20  2 2ud00 200  -0.49511 0.24514
              29  2 u2d00 200   0.46904 0.22000

        Natural orbitals and occupation numbers for root 1
        sym 1:   1.999604
        sym 2:   1.991918  1.943992  0.097398  0.000219  0.000640
        sym 4:   1.904095  0.061524  0.000611
        Natural orbitals and occupation numbers for root 2
        sym 1:   1.999436
        sym 2:   1.947529  1.248261  0.788864  0.028171  0.000731
        sym 4:   1.617765  0.032985  0.336259
        Natural orbitals and occupation numbers for root 3
        sym 1:   1.999273
        sym 2:   1.926567  1.085938  0.128802  0.904415  0.000774
        sym 4:   1.805386  0.141116  0.007730
        Natural orbitals and occupation numbers for root 4
        sym 1:   1.999591
        sym 2:   1.938931  1.828828  0.185815  0.001667  0.027931
        sym 4:   1.100050  0.074750  0.842438
        Natural orbitals and occupation numbers for root 5
        sym 1:   1.999251
        sym 2:   1.935074  1.086440  0.103317  0.001139  0.911640
        sym 4:   1.854839  0.074961  0.033340
        Natural orbitals and occupation numbers for root 6
        sym 1:   1.999766
        sym 2:   1.874358  1.484874  1.099307  0.004906  0.008790
        sym 4:   1.285113  0.235809  0.007076

.. Note: contains a nbsp

We have only included the configurations with weights larger than 10%.
Root one corresponds to the closed-shell ground state. To understand
the character of the states one must also analyze the orbitals,
remembering that the active orbitals are not ordered within the
active space.

The following output shows the coefficients of the diffuse functions
(center X) which appear in the |molcas| output.
Active orbitals two, three, and six in symmetry 2 are valence orbitals
(they have main contributions from the other functions not printed
here) and orbitals four and five are Rydberg orbitals. It is usual
that they appear as mixed orbitals (3p--3d here) but this mixing has no
consequences on the excitation energies. This is also the reason why
the Rydberg states appear not as clearly singly configurational
states but mixed as in root 5 (see above).

.. index::
   single: Orbitals; Rydberg
   single: Rydberg functions

::

     Molecular orbitals for symmetry species 2

     ORBITAL       2        3        4        5        6
     ENERGY      .0000    .0000    .0000    .0000    .0000
     OCC. NO.   1.8923   1.4570    .4122    .1674    .1689

  19 X  2px     -.0203    .0055   -.0082    .8091    .4535
  20 X  3d1+     .0064   -.0037    .0369    .4430  -1.0132

     Molecular orbitals for symmetry species 4

     ORBITAL       1        2        3
     ENERGY      .0000    .0000    .0000
     OCC. NO.   1.5865    .1722    .1439

  15 X  3d2-     .0032    .5171    .9600

.. Note: contains a nbsp

Both by looking at the configurations and the occupation numbers
we can identify the states. Root two has a main configuration described
by an excitation 3\ |bo| |->| 4\ |bo|. As 4\ |bo| is a valence orbital,
the resulting state will also be a valence state. Root three, on the
contrary, has a main configuration 3\ |bo| |->| 5\ |bo|, and 5\ |bo| is a
Rydberg orbital. 3\ |bo| is the HOMO\ |-|\1 orbital, therefore we can expect the
state represented by root three to be the HOMO\ |-|\1 |->| 3\ |px| Rydberg
state. So, why does configuration 3\ |bo| |->| 5\ |bo| contribute 21% to this
wave function
if a Rydberg state is just a singly excited state?. The answer is in the
composition of the orbitals. Orbitals four and five are a mixture
of |px| and |dxz|, and the configurational description must reflect that.

In summary we can make a initial classification of the states:

.. index::
   single: Excited states; Thiophene

| Root 1: Ground state
| Root 2: Valence :math:`\pi\to\pi^*` state
| Root 3: Rydberg 3\ |bo|\ |->|\3\ |px| state
| Root 4: Rydberg 3\ |at|\ |->|\3\ |dxy| state
| Root 5: Rydberg 3\ |bo|\ |->|\3\ |dxz| state
| Root 6: Valence :math:`\pi\to\pi^*` state

.. index::
   single: Orbitals; Valence–Rydberg mixing

Orbital two of symmetry 4 also deserves attention. It has large
contributions from the diffuse functions, although the remaining non-printed
coefficients are even larger. It is an orbital of mixed
valence--Rydberg character. This can affect the description of the valence
states. In the present system the problem is minor because the orbital
does not strongly participate in the description of the valence states
as it is shown by the configurations and the occupation numbers, but
in other systems the effect is going to be larger as we shall show later.

One important difference between valence and Rydberg states is the diffuse
character of the latter. We can analyze the orbital extension of the states.
Valence states have an orbital extension (second Cartesian moment)
similar to the ground state extension. Rydberg states, on the contrary,
should have a diffuse character. Additionally we can also study the
Mulliken population analysis. Both appear in the RASSCF output.

.. index::
   single: Properties; Mulliken analysis
   single: Mulliken analysis

::

       Mulliken population Analysis for root number: 1

       Gross atomic populations per centre and basis function type

                S1      C1      C2      H1      H2      X
       Total 15.8153 12.3470 12.2660  1.6887  1.8021   .0809

       Expectation values of various properties for root number: 1

   2-nd Cartesian moments: origin at (   .00000000,   .00000000,  2.15947162)
  ----------------------------------------------------------------------------
  Component                            XX              YY              ZZ
  Total                      -30.24626427    -21.54920631    -24.73702724

       Mulliken population Analysis for root number: 2

       Gross atomic populations per centre and basis function type

                S1      C1      C2      H1      H2      X
       Total 15.6548 12.3730 12.1962  1.6914  1.8015   .2831

       Expectation values of various properties for root number: 2

   2-nd cartesian moments: origin at (   .00000000,   .00000000,  2.15947162)
  ----------------------------------------------------------------------------
  Component                            XX              YY              ZZ
  Total                      -42.75835009    -28.13902538    -28.72863222

       Mulliken population Analysis for root number: 4

       Gross atomic populations per centre and basis function type

                S1      C1      C2      H1      H2      X
       3d2-    .0334   .0306   .0413   .0000   .0000   .9662
       Total 15.5924 11.8522 12.0083  1.6814  1.7986  1.0671

       Expectation values of various properties for root number: 4

   2-nd cartesian moments: origin at (   .00000000,   .00000000,  2.15947162)
  ----------------------------------------------------------------------------
  Component                            XX              YY              ZZ
  Total                      -89.85913318    -76.33249740    -44.45493589

       Mulliken population Analysis for root number: 6

       Gross atomic populations per centre and basis function type

                S1      C1      C2      H1      H2      X
       Total 15.6154 12.4779 12.3182  1.6946  1.8028   .0911

       Expectation values of various properties for root number: 6

   2-nd cartesian moments: origin at (   .00000000,   .00000000,  2.15947162)
  ----------------------------------------------------------------------------
  Component                            XX              YY              ZZ
  Total                      -31.85163136    -24.13169375    -26.69322385

.. Note: contains a nbsp

The Mulliken analysis provides us with the charge distribution per atom
and basis function. If we have used for the Rydberg states singly centered
Rydberg functions we can observe a population close to one on the X center.
This is what happened in root four (see above). In addition we can see that
the electron is placed in the 3d2\ |-| (3\ |dxy|) Rydberg orbital, confirming the
character of the state. The orbital extension is undoubtedly much larger
in the fourth root than in the ground state. The second and sixth roots however have
a much more compact description, especially the sixth, and they have low populations
on center X. The second root is somewhat more diffuse but it can be still considered
a clear valence state with minor Rydberg mixing.

.. index::
   single: RASSCF; Roots
   single: RASSCF; Average states

It is very important to ensure that
the relevant states of the symmetry are included in the CASSCF calculation. This
may mean performing different experiments by increasing the number of roots
and analyzing the results. Valence states are specially sensitive to this
because they are high roots at the CASSCF level. Take for instance
the sixth root. At the CASSCF level, it is 1.35 eV higher in energy than its
preceding root. It could happen that other close Rydberg states or even
valence states (such as mainly doubly excited states) were lower at this
level of calculation. It can be also helpful to analyze the transition moment
to be sure that the intense valence states are present in the set of computed
states.

.. compound::

  The RASSCF inputs for the remaining states replace the following keywords: ::

    &RASSCF
    Title
     Thiophene. pipi  1B2 states
    Symmetry
        3
    CiRoot
    5 5
    1 2 3 4 5
    1 1 1 1 1
    ...
    End of Input
    >> COPY $Project.JobIph $CurrDir/$Project.1B2.JobIph

  ::

    &RASSCF
    Title
     Thiophene. npi  1B1 states
    Symmetry
        2
    CiRoot
    1 1
    1
    ...
    End of Input
    >> COPY $Project.JobIph $CurrDir/$Project.1B1n.JobIph

  ::

    &RASSCF &END
    Title
     Thiophene. npi  1A2 states
    Symmetry
        4
    CiRoot
    2 2
    1 2
    1 1
    ...
    End of Input
    >> COPY $Project.JobIph $CurrDir/$Project.1A2n.JobIph

  ::

    &RASSCF &END
    Title
     Thiophene. pisigma  1B1 states
    Symmetry
        2
    Ras2
        5    3    2    2
    CiRoot
    6 6
    1 2 3 4 5 6
    1 1 1 1 1 1
    ...
    End of Input
    >> COPY $Project.JobIph $CurrDir/$Project.1B1.JobIph

  ::

    &RASSCF &END
    Title
     Thiophene. pisigma  1A2 states
    Symmetry
        4
    Ras2
        5    3    2    2
    CiRoot
    6 6
    1 2 3 4 5 6
    1 1 1 1 1 1
    ...
    End of Input
    >> COPY $Project.JobIph $CurrDir/$Project.1A2.JobIph

  ::

    &RASSCF &END
    Title
     Thiophene. nsigma  1A1 states
    Symmetry
        1
    Ras2
        5    3    2    2
    CiRoot
    4 4
    1 2 3 4
    1 1 1 1
    ...
    End of Input
    >> COPY $Project.JobIph $CurrDir/$Project.1A1n.JobIph

  and use the saved :file:`JOBIPH` files subsequently.

.. index::
   single: RASSCF; Active space
   single: Orbitals; Reordering

We must ensure that the right orbitals are included into
the active space. For instance, computing the :math:`^1A_2` and
:math:`^1B_1` Rydberg states with the active space (5322) we
observe that one Rydberg orbital is absent from the active space
in both cases. For the :math:`^1A_2` state it was orbital 3\ |dyz|.
Instead, an extra-valence :math:`\sigma^*` orbital took its place and
therefore the sixth root of symmetry :math:`^1A_2` was not the expected
2\ |bo| |->| 3\ |dyz| Rydberg state. In this case we can reorder
the orbitals including the Rydberg state in the active space
and excluding the other orbital and make the calculation again.
Hopefully the new calculation will include the Rydberg state
into the selected roots. If not we can always increase the
number of roots or increase the active space to have both
orbitals included.

It is very important to remember that to compute energy differences
one must always use states computed using the same active
space. Therefore, if we are computing vertical excitation energies
we must have the ground state energy computed in all the different
active spaces employed. One can make the comparison using a ground
state computed in the average procedure or as a single root. They
do not differ significantly. For consistency, we will use a ground state
computed as a single root. Therefore we have to perform two CASSCF
calculations using the inputs where we replace: ::

  >> COPY $CurrDir/$Project.11A1.JobIph JOBIPH
  &RASSCF &END
  Title
   Thiophene. Ground state (1503)
  Symmetry
      1
  Ras2
      1    5    0    3
  CiRoot
  1 1
  1

::

  >> COPY $CurrDir/$Project.11Ar.JobIph JOBIPH
  &RASSCF &END
  Title
   Thiophene. Ground state (5322)
  Symmetry
      1
  Ras2
      5    3    2    2
  CiRoot
  1 1
  1

.. index::
   single: Program; CASPT2
   single: CASPT2
   single: CASPT2; Lroot
   single: RASSCF; JOBIPH file
   single: CASPT2; JOBIPH file

.. _TUT\:sec\:pt2out:

CASPT2 calculations
...................

Once the reference wave functions have been computed at the CASSCF level
we can perform the CASPT2 calculations. The :file:`JOBIPH` file from
each CASSCF calculation contains data that describes the state(s).
If several CASSCF states are present on a :file:`JOBIPH` file, then any
of this may act as root function for the CASPT2. The input to the CASPT2
must then tell which one of the states we want. In previous |molcas|
version the keyword :kword:`LROOt` was used. Although it will still
work, it has been substituted by the more convenient keyword :kword:`MULTistate`,
which allows now to perform Multi-State CASPT2 calculations. We will start
by discussing single state CASPT2 calculations: ::

  &CASPT2 &END
  Title
   caspt2 input
  MultiState
  1 1
  End of input

The CASPT2 calculation will be performed on the ground state
with the active space (1305), stored on the :file:`JOBIPH` file that
we named :file:`$Project.11A1.JobIph`.
The final full CASPT2 result is: ::

        Reference energy:        -551.4423376617
        E2 (Non-variational):       -.6341237973
        E2 (Variational):           -.6341237319
        Total energy:            -552.0764613935
        Residual norm:               .0000008080
        Reference weight:            .80657

.. Note: contains a nbsp

For a perfectly converged result, the two formulae used to compute E2 are
equivalent, but if there are (as is usually the case) a small residual
error in the CASPT2 equation system, then the variational result is much
more accurate. In particular, for numerical differentiation the variational
energy should always be used. If a level shift has been used, in order to
avoid singularities (see below), then the non-variational energy and the
variational one will differ. The former is the conventional E2 as obtained
with the modified (shifted) :math:`\hat{H}_0` operator, while the latter is a
corrected value very close to what would have been obtained with the unshifted
operator if the near-singular term had been removed. The latter energy is
the one that should normally be used.

.. index::
   single: CASPT2; Weight

.. compound::

  For the ground state with a reasonable active space, all coefficients in the
  first order wave function and all contributions to the second-order energy
  will be small. For excited states, large contributions may occur, and then the
  second-order perturbation treatment may be invalid. One criterion for a good
  calculation is that the reference weight should be close to that of the ground
  state. When this is not true, special remedies may be considered.
  For example, we compute the CASPT2 correction for the sixth root of symmetry
  one, using the :file:`JOBIPH` file called :file:`$Project.1A1.JobIph`. The input is: ::

    &CASPT2 &END
    Title
     caspt2 input
    MultiState
    1 6
    End of input

  and the result (always full CASPT2 results): ::

          Reference energy:        -551.1062184006
          E2 (Non-variational):       -.7460718503
          E2 (Variational):           -.7460719607
          Total energy:            -551.8520232128
          Residual norm:               .0000009146
          Reference weight:            .29470

We observe a low weight of 0.295 for the CASSCF reference,
compared to the value 0.807 in the ground state. The low weight for
the excited state is a warning sign: the second order treatment may
be invalid. However, if so, the problem is due to one or a few
specific terms in the first-order wave function.

.. index::
   single: CASPT2; Intruder states
   single: Intruder states

In the output, there is a section with warnings for
large contributions to the energy, low denominator values, or large
coefficients.::

  CASE  SYM   ACT IND   NON-ACT INDICES  DENOMINATOR  RHS value  COEFFICIENT CONTRIBUTION

  ATVX   2   Mu2.0001   Se2.007           .01778941  -.00706261   .72136097  -.00509469
  ATVX   2   Mu2.0001   Se2.009           .20859986   .03118841  -.14372642  -.00448260
  ATVX   4   Mu4.0001   Se4.004           .02156184  -.01357269  1.20409651  -.01634282
  AIVX   1   Mu1.0001   In1.010 Se1.014   .08105563   .00023689  -.00197645  -.00000047
  AIVX   1   Mu1.0001   In3.007 Se3.012   .28275882  -.02231776   .08282960  -.00184857

In CASPT2, the wave operator is a sum of two-electron excitations,
:math:`\sum C_{pqrs}\hat{E}_{pqrs}`, where the singlet excitation operator
:math:`\hat{E}_{pqrs}` is normal-ordered and summed over spin. The electrons
are transferred from :math:`s` to :math:`r` and from :math:`q` to :math:`p`.

.. index::
   single: CASPT2; Excitations

No one-electron excitations are used. This is not due to any approximation;
it is simply because, for a RASSCF root function with active electrons, the
single excitations are exact linear combinations of the double excitations.

The non-orthogonality, as well as the non-diagonal terms of the :math:`\hat{H}_0`, makes it
difficult (and to some extent irrelevant) to obtain a label that partitions the
wave function and correlation energy in terms of orbital indices of elementary
excitations. However, the CASPT2 program uses internally an orbital system that
diagonalizes part of the Fock matrix: the block diagonal part which does not
include coupling between inactive, active and virtual orbitals. The first-order
wave function, or equivalently the first-order wave operator, can be
subdivided into terms that are grouped into eight different cases. These are named
by four-letter combinations as follows. The letters A, B, C or D are used for
secondary (virtual) orbitals; T, U, V, or X for active ones, and I, J, K or L
for inactive orbitals. A case such as ATVX contains wave operator terms that
can be written as :math:`\hat{E}_{atvx}`, where :math:`a` is a virtual orbital and :math:`t`, :math:`v`,
and :math:`x` are active.

The first-order wave function can be subdivided into individual terms labeled
by the case (e.g. ATVX), the individual non-active orbital indices, and an
active superindex that labels a linear combination of terms with different
active orbital indices. The linear combination will "mix" all active indices or
index combinations within the case (with symmetry restrictions, if any) in such
a way that *the individual terms that are used internally in the CASPT2
programs are orthogonal, and they diagonalize the block-diagonal part of* :math:`\hat{H}_0`.

.. index::
   single: CASPT2; Denominators

Of course, the complete :math:`\hat{H}_0` is used to solve the CASPT2 equations, which
is why an iterative procedure is needed. However, in the diagnostic output above,
the "DENOMINATOR" value is that of the resolvent of the block-diagonal part of
:math:`\hat{H}_0`. However, for diagnostics, this is a good approximation. (That it is
not exact only shows by the fact that singularities in the energy do not
occur exactly when the "DENOMINATOR" reported is equal to 0.)

The orbitals are labeled by the symmetry type, a period, and then the ordering number
within that symmetry type. However, for clarity, it also is prefixed by the letters
"Fr", "In", "Ac", "Se" or "De" for frozen (uncorrelated), inactive, active,
secondary, and deleted orbitals. In the wave operator, the only possible orbital
labels are "In" and "Se".
The active superindex is given in formulae as :math:`\mu`, :math:`\nu`, etc. so it is
given a prefix "Mu".

Most of the cases are further subdivided into a plus and a minus linear combination
making altogether 13 cases. Thus, the BVAT case is subdivided into BVATP and BVATM,
containing terms of the type :math:`\hat{E}_{bvat} \pm \hat{E}_{avbt}`, respectively.
This has nothing to do with spin. It offers some technical advantages in the
equation solution.

.. table:: Labeling for the configurations in :program:`CASPT2`.
   :name: tab:pt2ex

   ======= === ============ ==== ============= ============ ==== =============
   Config.     Excitation 1                    Excitation 2
   ======= === =============================== ===============================
   VJTU        Inactive (J) |->| Active (V)      Active (U) |->| Active (T)
   VJTIP       Inactive (J) |->| Active (V)    Inactive (I) |->| Active (T)
   VJTIM       Inactive (J) |->| Active (V)    Inactive (I) |->| Active (T)
   ATVX          Active (T) |->| Secondary (A)   Active (X) |->| Active (V)
   AIVX        Inactive (I) |->| Secondary (A)   Active (X) |->| Active (V)
   |zws|   or:   Active (X) |->| Secondary (A) Inactive (I) |->| Active (V)
   VJAIP       Inactive (J) |->| Active (V)    Inactive (I) |->| Secondary (A)
   VJAIM       Inactive (J) |->| Active (V)    Inactive (I) |->| Secondary (A)
   BVATP         Active (V) |->| Secondary (B)   Active (T) |->| Secondary (A)
   BVATM         Active (V) |->| Secondary (B)   Active (T) |->| Secondary (A)
   BJATP       Inactive (J) |->| Secondary (B)   Active (T) |->| Secondary (A)
   BJATM       Inactive (J) |->| Secondary (B)   Active (T) |->| Secondary (A)
   BJAIP       Inactive (J) |->| Secondary (B) Inactive (I) |->| Secondary (A)
   BJAIM       Inactive (J) |->| Secondary (B) Inactive (I) |->| Secondary (A)
   ======= === ============ ==== ============= ============ ==== =============

For more details see Refs. :cite:`Andersson:90,Andersson:92a,Andersson:92e`

The first configuration shown in the thiophene output involves the excitation
from the active space to the secondary orbital, which is orbital nr
seven of symmetry two (Se2.007). The denominator value for this
configuration is close to zero (0.01778941). This is an energy difference,
in the :math:`\hat{H}_0` approximation. Thus the root state, and some
eigenstate of :math:`\hat{H}_0` in the interacting space, have almost the same
energy value.

Such states, that were not included in the CASSCF configuration interaction
but have energies within the range of the lowest CAS states, cause frequent
problems in excited state calculations, since they often give
small denominators and even, at particular geometries, singularities. We
call these states intruders, by analogy to a similar phenomenon in multi-state
perturbation theory.
A calculation of excited states by means of a perturbation theory
based on an active space has to deal with the problem of intruder states.
This is especially common when large and diffuse basis
sets, such as the Rydberg functions, are included in the calculations.

In this example, the coefficient to the
first order wave function is large (0.72136094). So is
the contribution to the second order energy (\ |-|\0.00509469 |Eh|),
|-|\0.14 eV. Even worse is the situation for the third term printed
involving the fourth orbital (secondary) of symmetry four
with an energy contribution of 0.44 eV. The analysis of the secondary
orbitals 7\ |bo| and 4\ |at| (they are the first virtual orbital of their
symmetry) indicates that they are extremely diffuse orbitals with
large Rydberg character. Remember that the subspaces we are
using are: frozen (4130), inactive (6040), and active (1503).

This is not the case in the other configurations shown. First we
have other ATVX terms including the excitation to the secondary
orbital Se2.009. Also we have an AIVX term, involving
the excitation from inactive In3.007 to secondary Se3.012. Their
contributions to the second order energy, |-|\0.00448260 and |-|\0.00184857,
respectively, are not caused by accidental near degeneracies in the value of
the denominator. The orbitals involved are not of Rydberg character either.
We have finally included as an example the excitation AIVX involving
the excitation from In1.010 to Se1.014. Although it has a small value
for the denominator, its contribution to the second order energy is
very small and therefore it does not represent an important problem.

Intruders can be eliminated by including sufficiently many orbitals in
the active space. When this is a reasonable alternative, it is the preferred
solution. Limitations in the number of active orbitals
can make this approach impractical. However, especially when intruders
have clear Rydberg character, their effect on the second-order energy is
often small, except perhaps in a small range of geometries around a singularity
due to accidental degeneracy. In this common situation, two other remedies
are available: shifting the :math:`\hat{H}_0` Hamiltonian, or deleting virtual
orbitals. These remedies will be described in some detail in the following.

.. index::
   single: Intruder states
   single: CASPT2; Intruder states
   single: CASPT2; Level-shift
   single: CASPT2; Shift
   single: LS-CASPT2

In order to obtain continuous potential energy functions, one cannot use a
case-by-case approach, such as deleting an orbital. However, the :math:`\hat{H}_0`
can be modified in such a way as to eliminate weak singularities.
A well-tested method is a level-shift technique called
LS-CASPT2 :cite:`Roos:96b,Roos:95b`.
A constant parameter is added to the external part of the zeroth-order
Hamiltonian. Any denominator close to zero is thus shifted away from zero, and
does not produce any singular term. Of course, in a worst-case scenario, it
might happen that some other denominator, previously non-zero, is shifted to
come close to zero. In general,
it is the higher excited states, in combination with large diffuse basis sets
and exploration of a large range of geometries, that is the greatest risk for
troublesome intruders.

There is also a new, less tried technique, called the imaginary shift
method :cite:`Forsberg:96`. Here, the use of an imaginary shift value (but
taking the real part of the computed correlation energy) offers some
advantage, since an imaginary shift cannot introduce new singularities.

.. compound::

  With either of the level shift methods, the (2nd order) correlation energy :math:`E_2`
  and the (1st order) wave function will depend on
  the level shift used. A correction of therefore applied, whereby in practice
  this dependence is made small, except of course for the spurious term that has
  disappeared. The corrected energy is in fact computed by using Hylleraas' 2nd-order
  variational formula to evaluate :math:`E_2`, with the *unshifted* :math:`\hat{H}_0`,

  .. math:: E_2 = 2 \braopket{\Psi_1}{\hat{H}}{\Psi_0} + \braopket{\Psi_1}{\hat{H}_0}{\Psi_1}

  which we call the *variational* :math:`E_2` in the output listing.

To minimize the effect on relative energies, we recommend that the same level shift is
used for all states and geometries, if possible. This may require some
experimenting. A criterion on absence of disturbing intruders is that
the weight of the reference wave function should be roughly the same in
all calculations. Without shift, a difference of up to 10% between the weights
of the ground and an excited state can be acceptable (that is, the excitation energy
is accurate enough) in a :program:`CASPT2` calculation without level shift.
Using level shift, this should be adjusted to find a better match of reference weights.
A detailed explanation of how to use the level-shift technique has been
published :cite:`Roos:96a`. Here we will simply summarize the main aspects.

Using the same :file:`JOBIPH` file as before we perform a new CASPT2 calculation
using the input: ::

  &CASPT2 &END
  Title
   caspt2 input
  MultiState
  1 6
  Shift
  0.1
  End of input

A level-shift of 0.1 |Eh| has been introduced as a separation of the
eigenvalues of the zeroth-order Hamiltonian. The final energy is then
corrected, and the result is: ::

        Reference energy:        -551.1062184006
        E2 (Non-variational):       -.6921992859
        Shift correction:           -.0334372801
        E2 (Variational):           -.7256365659
        Total energy:            -551.8315878181
        Residual norm:               .0000003986
        Reference weight:            .74942

  CASE  SYM   ACT IND   NON-ACT INDICES  DENOMINATOR  RHS value  COEFFICIENT CONTRIBUTION

  ATVX   2   Mu2.0001   Se2.007           .01778941  -.00706261   .06072347  -.00042887
  ATVX   2   Mu2.0001   Se2.009           .20859986   .03118841  -.09700134  -.00302532
  ATVX   4   Mu4.0001   Se4.004           .02156184  -.01357269   .11838970  -.00160687
  AIVX   1   Mu1.0001   In3.007 Se3.012   .28275882  -.02231776   .05918658  -.00132091

.. Note: contains a nbsp

Several details come to our attention. Firstly, the final CASPT2 energy is
higher than the result with level-shift 0.0. This is because the introduction
of the parameter decreases the amount of dynamical correlation included.
Secondly, the weight of the reference function has increased greatly, from
0.29 to 0.74, meaning that the most important intruder states have been
removed from the treatment. Finally, we can observe the new contributions
of the printed configurations to the second order energy. Configurations
involving excitations to the 7\ |bo| and 4\ |at| orbitals have drastically decreased
their contributions, proving that the previous contributions
were due to degeneracies in the denominators. However, the other two
configurations remain almost as they were before, only slightly
decreasing their contributions.

Now we use a value for the level-shift parameter of 0.2 |Eh|: ::

        Reference energy:        -551.1062184006
        E2 (Non-variational):       -.6619040669
        Shift correction:           -.0557159229
        E2 (Variational):           -.7176199898
        Total energy:            -551.8235712419
        Residual norm:               .0000009298
        Reference weight:            .78212

  CASE  SYM   ACT IND   NON-ACT INDICES  DENOMINATOR  RHS value  COEFFICIENT CONTRIBUTION

  ATVX   2   Mu2.0001   Se2.007           .01778941  -.00706261   .03193515  -.00022555
  ATVX   2   Mu2.0001   Se2.009           .20859986   .03118841  -.07304944  -.00227830
  ATVX   4   Mu4.0001   Se4.004           .02156184  -.01357269   .06238180  -.00084669
  AIVX   1   Mu1.0001   In3.007 Se3.012   .28275882  -.02231776   .04673419  -.00104300

The observed tendencies are maintained. Finally, a value of 0.3 |Eh|: ::

        Reference energy:           -551.1062184006
        E2 (Non-variational):          -.6347955450
        Shift correction:              -.0735679820
        E2 (Variational):              -.7083635270
        Total energy:               -551.8145819276
        Residual norm:                  .0000006328
        Reference weight:               .80307

  CASE  SYM   ACT IND   NON-ACT INDICES  DENOMINATOR  RHS value  COEFFICIENT CONTRIBUTION

  ATVX   2   Mu2.0001   Se2.007           .01778941  -.00706261   .02173413  -.00015350
  ATVX   2   Mu2.0001   Se2.009           .20859986   .03118841  -.05865340  -.00182931
  ATVX   4   Mu4.0001   Se4.004           .02156184  -.01357269   .04240583  -.00057556
  AIVX   1   Mu1.0001   In3.007 Se3.012   .28275882  -.02231776   .03862959  -.00086213

The contributions to the energy are much lower for each increase of the
parameter, but we must never forget that we are losing dynamical correlation
with the increase of the level-shift factor. In a calculation of excitation
energies that means that the resulting excitation energies become larger each time
(dynamical correlation is larger in the excited state).
Therefore, the level-shift parameter must be
set to the lowest possible value which solves the intruder state problems.
In practice it is then convenient to scan all the valence states for several
values of the parameter and look for two factors:

#. Reference weight as close as possible to the ground state reference weight
   with the same level shift parameter (LS).
#. Excitation energies (ES) as stable as possible with the increment of the
   level-shift parameter (LS).

We now compute the ground state (GS) also for the level-shift values of 0.1, 0.2,
and 0.3, and compare the excitation energies :math:`\Delta E` (always between states
computed with the same parameter):

.. table:: Excitation energies and reference weights of thiophene
           for different level shift values.
   :name: tab:thiols

   ===================== ===================== ===================== =====================
   LS (|Eh|)             :math:`\Delta E` (eV) weight GS             weight ES
   ===================== ===================== ===================== =====================
   0.0                   6.11                  0.81                  0.29
   0.1                   6.64                  0.82                  0.75
   0.2                   6.79                  0.83                  0.78
   0.3                   6.89                  0.84                  0.80
   ===================== ===================== ===================== =====================

After checking the remaining states we conclude that a level shift
of 0.1 |Eh| is enough for our purposes. However the results
seem to be too unstable with respect to the increase of the level-shift
parameter. As our active space only comprises nine orbitals, we can consider the
possibility of increasing it by including two more active orbitals in
symmetries |bo| and |at|. In this way we minimize the intruder states
problems in the best way, by introducing extra (not diffuse hopefully)
orbitals. This will increase the accuracy.

.. index::
   single: CASPT2; Intruder states

The introduction of a (real) level-shift parameter does not automatically
remove intruder state problems. It happens that a shift leads to more
severe problems that those observed without level-shift. Examples
and further explanations are given in e.g. ref. :cite:`Roos:96a`.
In such a case is may be possible to find a range of level-shift values
where none of the computed states present intruder state problems.
In a few cases we have found it necessary to use a shift larger than
0.3 |Eh|. Another solution is to try an imaginary shift. This option has
not been extensively investigated yet.

Consider a situation like the following: ::

  CASE  SYM   ACT IND   NON-ACT INDICES  DENOMINATOR  RHS value  COEFFICIENT CONTRIBUTION

  ATVX   2   Mu2.0001   Se2.004          -.30281661  -.00194108  -.37224517   .00072256

This is a calculation performed using level shift of 0.3 |Eh|. (The approximate
denominator printed in the listing is that *without* the added shift).
We have added
the level shift to solve intruder states problem in other states, but we
should use the same technique for all the computed states for consistency
reasons (of course always using a ground state computed with the same
level shift value). We find, however, that
the weight of the CASSCF reference function is lower in the case with
level shift 0.3 |Eh| (0.61) than in the case without level shift (0.69).
In this state we have a denominator with
a value close to |-|\0.3 |Eh|. As the level shift we apply is a positive
quantity (0.3 |Eh|) added to this denominator, we have created a problem
by decreasing the denominator to a value close to zero. The coefficient
of the configuration increases, which is reflected in the
contributions to the second-order energy. Therefore, before applying
any level shift, it is wise to check the values of the most important
denominators to see if any of them is going to be close to the value
of the applied level shift. In those situations we should set the level
shift to another value. Sometimes the consequences for the final energy
are small (here for instance) but this is not always the case
(see ref. :cite:`Roos:96a`).

.. index::
   single: Orbitals; Deleted

It is also possible to delete virtual orbitals. This is occasionally
used, e.g. when using other types of basis sets than ANO's, in order
to delete virtual orbitals that are core-correlating.
The procedure to do that is to take an orbital file, such as that
produced by SCF or RASSCF, and edit it by hand and then using it as
:file:`INPORB` file in the RASSCF step. The orbitals one wants
to delete are placed at the end of their symmetry group, and the
keyword :kword:`DELEted` in used the RASSCF input, indicating
how many orbitals are going to be deleted by symmetry. The program will
ignore the deleted orbitals, both in RASSCF and the subsequent CASPT2 steps.
To obtain accurate energy differences
it is necessary to use the same set of initial orbitals and recompute the
ground state (or the state one is comparing with) with the same number
of deleted orbitals.

When the above scheme is used in order to try to eliminate intruders in
CASPT2, the best way is if the :file:`INPORB` can be prepared from the
CASPT2 calculation where the intruder problem occurred.

For that calculation, the natural orbital analysis that
follows the CASPT2 calculation shows up a virtual orbital with abnormally
large occupation number and diffuse character. Use an editor to move this orbital
to the end of the orbital file, and use it as :file:`INPORB`.
When the calculation is repeated, intruders with this orbital heavily
populated have been eliminated.
Occasionally, several orbitals need to be removed.

The deletion of virtual orbitals works best at single-geometry
calculations, such as obtaining the vertical electronic spectrum.

.. index::
   single: MSCASPT2

Let us focus on the Multi-State CASPT2 type of calculations. The original
reference :cite:`Finley:98b` should be carefully read before using the method.
This multidimensional perturbative approach considers the coupling of a number
of CASPT2 states, a condition which is crucial to solve certain problems such
as adiabatic crossing among states, strong valence--Rydberg situations, etc.
The treatment is performed for a number of roots of the same symmetry provided
they originate from a previous State-Average CASSCF calculation, that is, the
:program:`CASPT2` program will use the binary :file:`JOBIPH` file from a
previous SA-CASSCF calculation, for instance, the six roots :math:`^1A_1` CASSCF
calculation in thiophene. The corresponding :program:`CASPT2` input to treat
simultaneously the six states will be: ::

  &CASPT2 &END
  Title
   mscaspt2 input
  MultiState
  6 1 2 3 4 5 6
  Shift
  0.3
  End of input

A level shift parameter of 0.3 |Eh| has been selected for comparison with the
previous calculations. The program creates a new
binary file, :file:`JOBMIX`, which contains the newly generated Perturbatively
Modified (PM) CASSCF wave function.

Using the previous input, the :program:`CASPT2` module will perform in a single
run six consecutive single-root CASPT2 calculations for each one of the CASSCF
states. At the end of each of the calculations the contributions to the Hamiltonian
coupling elements between the computed and the remaining states will be printed.
After computing the six CASPT2 roots, the MS-CASPT2 treatment will be performed.
First, the effective Hamiltonian matrix, asymmetric and symmetric, is printed. ::

    Effective Hamiltonian matrix (Symmetric):


                  1                2              3               4               5
    1        -.07013926
    2        -.01263691       .12976380
    3         .00071175       .01001560       .18051855
    4         .00509735       .00990244      -.00321669       .19922802
    5         .00607124       .00070650      -.00129815      -.00225583       .21601193
    6         .01998132       .02350235      -.00771000      -.01037132      -.00264941

                  6
    1         .18541807

.. Note: contains a nbsp

Notice that the diagonal elements of the matrix correspond to the single root
CASPT2 state energies, where some quantity, 551.0 |Eh| here, has been added to get a
better print of the output. Following, the eigenvalues and eigenvectors of the
diagonalized matrix are obtained: ::

    Energies and eigenvectors:

      -552.07305076  -551.88140802  -551.81866833  -551.80756578  -551.79500203

          .99308520     -.10131857      .01038991      .05207094     -.02055799
          .07343489      .90295279      .31190606      .28061095     -.05245262
         -.00869768     -.19493901      .90626880     -.37241673      .03796203
         -.02478279     -.15572120      .13596794      .50373403      .83205915
         -.02204833     -.01553573      .05330075      .08679334      .05789830
         -.08492920     -.33454317      .24485766      .72011863     -.54745806

      -551.78350398

          .01655899
         -.02245882
         -.02155609
         -.10285444
          .99274682
         -.05129770

.. Note: contains a nbsp

The eigenvalues correspond to the final MS-CASPT2 energies, while the eigenvectors
describe the combination of the coupled CASPT2 state which give rise to the final
MS-CASPT2 states. **Important:** Notice that the states are written in an increasing
energy order, and therefore they do not, in general, correspond to the order
obtained in the previous SA-CASSCF calculation. For instance, the MS-CASPT2
state number six, energy |-|\551.78350398 au, mainly correspond to the fifth state
of the previous calculation. It is very important to remember that the final
states are linear combinations of the preceding ones, and therefore a one to
one correspondence is hardly possible. In the present example most of the MS-CASPT2
states have a strong weight in just one of the preceding states, but this is not
the case in many situations. Following in the output, a printing of the new
wave function is obtained. It corresponds to linear combinations of the SA-CASSCF
CI wave functions, obtained in the basis of the previous CASSCF averaged orbitals. ::

    The CI coefficients for the MIXED state nr.   1
   ----------------------------------------------------------------------------
   CI COEFFICIENTS LARGER THAN 0.36
    Occupation of active orbitals, and spin coupling
    of open shells. (u,d: Spin up or down).
     Conf Occupation        Coef          Weight
       11  2 22000 200    .960835       .923204

    The CI coefficients for the MIXED state nr.   2
   ----------------------------------------------------------------------------
   CI COEFFICIENTS LARGER THAN 0.36
    Occupation of active orbitals, and spin coupling
    of open shells. (u,d: Spin up or down).
     Conf Occupation        Coef          Weight
       20  2 2ud00 200    .856751        .734023

    The CI coefficients for the MIXED state nr.   3
   ----------------------------------------------------------------------------
   CI COEFFICIENTS LARGER THAN 0.36
    Occupation of active orbitals, and spin coupling
    of open shells. (u,d: Spin up or down).
     Conf Occupation        Coef          Weight
       85  2 2u0d0 200    .764848        .584993
       86  2 2u00d 200    .507350        .257404

    The CI coefficients for the MIXED state nr.   4
   ----------------------------------------------------------------------------
   CI COEFFICIENTS LARGER THAN 0.36
    Occupation of active orbitals, and spin coupling
    of open shells. (u,d: Spin up or down).
     Conf Occupation        Coef          Weight
        1  2 22200 000   -.368003        .135427
       14  2 22000 u0d    .732276        .536229

    The CI coefficients for the MIXED state nr.   5
   ----------------------------------------------------------------------------
   CI COEFFICIENTS LARGER THAN 0.36
    Occupation of active orbitals, and spin coupling
    of open shells. (u,d: Spin up or down).
     Conf Occupation        Coef          Weight
        1  2 22200 000    .416925        .173826
       12  2 22000 ud0    .549793        .302272
       14  2 22000 u0d    .455052        .207072

    The CI coefficients for the MIXED state nr.   6
   ----------------------------------------------------------------------------
   CI COEFFICIENTS LARGER THAN 0.36
    Occupation of active orbitals, and spin coupling
    of open shells. (u,d: Spin up or down).
     Conf Occupation        Coef          Weight
       85  2 2u0d0 200    -.517972       .268295
       86  2 2u00d 200     .776117       .602358

.. Note: contains a nbsp

.. index::
   single: PMCASSCF wave functions

The comparison of the present wave functions, that will be hereafter
called Perturbatively Modified (PM) CASSCF wave functions, and the
previous CASSCF wave functions leads to several conclusions. Remember
that the orbital basis has not changed, therefore those mixing related
to the orbitals are not going to disappear. For instance, state number
three will still be formed by two configurations, because the Rydberg
3px character is still delocalized between orbitals 5 and 6 or symmetry
\bo. However the character of the second root has changed dramatically.
Now one single configuration describes the state, which has acquired a
very clear valence character. The previous mixing with a Rydberg-like
configuration has disappeared. It is illustrative to carry out
an additional analysis of the obtained states using the generated
file :file:`JOBMIX` as input file to perform
a :program:`RASSI` calculation, in which new PM-CASSCF properties
for the states will be obtained. Even when the changes in energies
are small, changes in the properties can be considerable.
:program:`RASSI` provides different types of matrix elements
(see next section), and dipole moments, transition dipole moments
and their directions, and orbital extensions (all of them available
from the :program:`RASSI` output) will be crucial for our purposes
in the study of excited states.

Finally, it is necessary to remember that the extent of the MS interaction
relies on the mixing of the previous states. This depends on different
factors. The basis sets is one of them. The use of one or other atomic
basis set to describe the diffuse functions may lead to different answers.
It is not uncommon that CASPT2 results with different diffuse basis sets
give different answers due to different extents of the valence--Rydberg
mixing. It will be necessary to perform final MS-CASPT2 calculations.
Those will change the CASPT2 result in some cases, but it will be
unaffected in other cases. Another effect comes from the use
of the level shift. The use of MS-CASPT2 does not prevent or
affect the extent of the intruder effects. Remember that this effect
is already included both in the diagonal terms of the effective
Hamiltonian as in the non-diagonal coupling terms. Still a careful checking
of different LS values and how they affect the CASPT2 values must be
performed, and the final MS-CASPT2 results should be those in which
the effect of the intruder states is small, always trying to use as low level
shift values as possible. An alternative is to use an imaginary level shift.
Finally, the extent of the off-diagonal coupling elements and its asymmetric
character introduce further inaccuracies in the treatment. In most cases the
proper enlargement of the active space diminishes most of the spurious
effects and increases the accuracy.

.. index::
   single: Properties; Transition dipole moments
   single: Transition dipole moments
   single: Program; RASSI
   single: RASSI

.. _TUT\:sec\:rassi_thio:

Transition dipole moment calculations
.....................................

One powerful tool included in the |molcas| package is the :program:`RASSI` program.
RASSI (RAS State Interaction) forms matrix elements of the Hamiltonian
and other operators in a wave function basis which consists of
individually optimized CI expansions from the RASSCF program.
It also solves the Schrödinger equation within the space of these
wave functions. In spectroscopy we need to compute the matrix elements
of a one-electron operator such as the dipole transition moment
to obtain the intensity of the transitions. In an absorption process
this means computing the interaction of the ground state with the
excited states. RASSI will compute all matrix elements among the
states provided they have been computed with the number of inactive
and active orbitals, and using the same basis set.
The transition dipole moments are computed using the length
representation.

.. compound::

  In our example we have used two different active spaces.
  We therefore need to perform at least two RASSI calculations.
  First we will compute the interaction of the ground
  state :math:`1^1A_1` (computed as single root), with the :math:`\pi\to\pi^*`
  :math:`^1A_1` and :math:`^1B_2` excited states. We should link the corresponding
  :file:`JOBIPH` files:

  .. index::
     single: RASSI; JOBIPH file

  ::

    ln -fs $Project.11A1.JobIph JOB001
    ln -fs $Project.1A1.JobIph JOB002
    ln -fs $Project.1B2.JobIph JOB003

  and use the RASSI input file: ::

    &RASSI &END
    Nrofjobiphs
     3 1 5 5
      1
      2 3 4 5 6
      1 2 3 4 5
    End of input

As we are using states that are not orthogonal (this is the case
among the :math:`1^1A_1` ground state computed as a single root and
the other :math:`^1A_1` states) we must take the matrix elements
of the transition dipole moment computed after the transformation
to the eigenbasis; the second time they appear in the
output: ::

   PROPERTY: MLTPL  1   COMPONENT:   2
   ORIGIN    :  .00000000D+00  .00000000D+00  .00000000D+00
   STATE     :       1              2              3              4

       1        .00000000D+00  .00000000D+00 -.43587844D+00  .00000000D+00
       2        .00000000D+00  .00000000D+00 -.10019699D+01  .00000000D+00
       3       -.43587844D+00 -.10019699D+01  .00000000D+00 -.46859879D+00
       4        .00000000D+00  .00000000D+00 -.46859879D+00  .00000000D+00
       5        .90773544D-01  .75718497D-01  .00000000D+00  .27645327D+00
       6        .00000000D+00  .00000000D+00  .41227462D+01  .00000000D+00
       7        .00000000D+00  .00000000D+00  .89741299D+00  .00000000D+00
       8       -.16935368D+00  .15487793D+01  .00000000D+00 -.41013917D+01
       9        .81381108D+00  .79559359D+00  .00000000D+00 -.88184724D-01
      10        .00000000D+00  .00000000D+00 -.43659784D+00  .00000000D+00
      11        .13520301D+01  .50454715D+00  .00000000D+00  .56986607D-01

  ...

   PROPERTY: MLTPL  1   COMPONENT:   3
   ORIGIN    :  .00000000D+00  .00000000D+00  .22419033D+01
   STATE     :       1              2              3              4

       1        .28126942D+00 -.92709234D+00  .00000000D+00  .11876829D+00
       2       -.92709234D+00  .26218513D+00  .00000000D+00  .14100968D+00
       3        .00000000D+00  .00000000D+00  .52558493D-01  .00000000D+00
       4        .11876829D+00  .14100968D+00  .00000000D+00  .36996295D+00
       5        .00000000D+00  .00000000D+00 -.43197968D+01  .00000000D+00
       6       -.15470487D+00 -.42660550D+00  .00000000D+00  .94593876D+00
       7       -.18676753D-01  .18738780D+01  .00000000D+00 -.37737952D+01
       8        .00000000D+00  .00000000D+00 -.28182178D+00  .00000000D+00
       9        .00000000D+00  .00000000D+00  .38253559D+00  .00000000D+00
      10        .12859613D+01  .48476356D+00  .00000000D+00  .35525361D+00
      11        .00000000D+00  .00000000D+00 -.39325294D-01  .00000000D+00

.. Note: contains a nbsp

.. index::
   single: RASSI; Symmetry

We have a symmetric matrix containing the results. The matrix elements
corresponding to the interaction of the first state in the input
(ground state) and the remaining states appear both in the first
column and in the first row (only partially printed here). Remember
that the transition dipole moment (TDM) matrix elements are determined by the symmetry.
The matrix element :math:`\braopket{^1A_1}{\text{TDM}}{^1A_1}` will be zero for the
:math:`x` and :math:`y` components of TDM, and non-zero otherwise.
The matrix element :math:`\braopket{^1A_1}{\text{TDM}}{^1B_2}` will be non-zero only
for the :math:`y` component of TDM. This is because the product
(wave function 1 |x| dipole moment component |x| wave function 2), if decomposed into
irreducible representations, must contain the
totally symmetric representation to have an allowed transition. In this simple case,
we can use a multiplication table for the irreps.
Thus, for instance, (:math:`^1A_1(z) \times \text{TDM}_y \times {}^1A_1(z)`) gives :math:`y`, which
does not belong to the totally symmetric representation. A look at the
character table and the behavior of the :math:`x`, :math:`y`, :math:`z` functions will give us the
information we need.

Therefore, in the component two (:math:`y`) of the transition dipole moment
matrix elements we have zero values for the interaction among :math:`^1A_1`
states and non-zero values for the interaction among :math:`^1A_1` and :math:`^1B_2`
states.

The :program:`RASSI` program in 6.0 and later versions of |molcas| will print the
oscillator strengths and the Einstein :math:`A` coefficients for all transitions. Also
the angles of the transition moment vectors to the coordinate axes will be
printed. In the calculation :program:`RASSI` will use the energies given as input, so be
careful to use the keywords :kword:`HDIAG` or :kword:`EJOB` to use energies which include dynamic
correlation.

We illustrate how the oscillator strengths are computed. The 11 states are
ordered by CASSCF energies. We focus on the valence states; firstly the fourth
and fifth :math:`^1B_2` states. Their transition dipole moment values in
atomic units are 0.81381108 and 1.3520301, respectively. The oscillator
strength is defined as:

.. index::
   single: Oscillator strength
   single: Properties; Oscillator strength

.. _eqn\:oscillator:

.. math:: f = \frac{2}{3} (\text{TDM})^2 \Delta E

The energy difference :math:`\Delta E` is the excitation energy expressed in atomic
units. The transition moments were computed by CASSCF. It is usually not practically
possible to compute them with dynamic correlation included, except if a common set
of orbitals are used. However, the CASSCF values are usually good enough.
(Exceptions occur, e.g. close to narrowly avoided crossings or conical intersections).
The excitation energies, on the other hand, are quite sensitive to dynamic
correlation.
Thus, it is a good approach to
use CASSCF TDMs and CASPT2 excitation energies. The values for the oscillator
strengths of the two :math:`^1B_2` valence states are 0.086 and 0.324, respectively.
The excitation energies are 5.31 and 7.23 eV, respectively. All data corresponds
to results obtained using the 0.1 |Eh| value for the level-shift parameter.

Remember that in other symmetries like :math:`C_{2h}` the :math:`^1B_2` states have
two components of TDM, :math:`x` and :math:`y`, for which the matrix elements with
respect to the ground state are non-zero. In this case the :math:`\text{TDM}^2` value
is computed as :math:`\text{TDM}_x^2 + \text{TDM}_y^2`. In those cases is is also possible to
compute the direction of the total TDM vector by taking their components and
compute the angle respect to any of the axis.

You will find the complete calculation of the absorption spectrum of thiophene
in reference :cite:`Serrano:93a`. You can observe that, despite there being
no level-shift technique used, the final results on the excitation energies
agree to within 0.1 eV to those shown here.

.. index::
   single: Valence states
   single: Rydberg states
   single: Basis set; Rydberg functions

Influence of the Rydberg orbitals and states. One example: guanine
------------------------------------------------------------------

Thiophene has a valence :math:`\pi`,\ :math:`\pi^*` orbital space small
enough to allow the simultaneous inclusion of all the corresponding Rydberg orbitals
into the active space (remember valence space (1302) + Rydberg spaces
(0201) or (4020)), but this is not always the case. In addition, the
valence--Rydberg mixing is not severe. This mixing is reflected
in the orbital extension or the population analysis. In difficult cases
valence and Rydberg orbitals mix, and then the configurations also mix.
Valence states become more diffuse and Rydberg states more compact.
Energetically this has minor consequences for the Rydberg states, which
can be computed using these CASSCF mixed wave functions. This is not
the case for the valence states. They are extremely sensitive to the
mixing. Therefore, if we do not observe clear and compact valence states
some mixing has occurred.

.. index::
   single: Guanine
   single: Excited states; Rydberg
   single: Active space

We consider
the example of the guanine molecule, the nucleic acid base monomer.
It is a system with 11 valence :math:`\pi`,\ :math:`\pi^*` orbitals which should be included
into the active space. It is a planar system
in the :math:`C_s` point group. Focusing only in the :math:`\pi\to\pi^*` states
we can label the active orbital space (0,11) where 0 is the number of
:math:`a'` orbitals and 11 the number of :math:`a''` orbitals. In :math:`C_s` symmetry the
Rydberg orbitals are distributed as (6,3), using the same labeling.
Therefore the calculation of the corresponding :math:`A'` states should
use the space (0,14) with 14 active electrons and a large number of
roots. This is a large calculation that one might want to avoid.
One can perform several
test calculations (maybe even RASSCF calculations) and find if
any orbitals can be excluded. The lowest occupied
:math:`\pi` orbital is a deep orbital which does not participate in the
lowest valence excited states and can be excluded from the active
space. Despite this exclusion, a (0,13) orbitals calculation is
still expensive. We can proceed in another way.
Consider the new valence space (0,10), and add only one more
orbital designed to include the first Rydberg orbital.
With this space of (0,11) orbitals and 12 active electrons we perform
a CASSCF including 6 roots.

.. figure:: guanine.*
   :name: fig:guanine
   :width: 50%
   :align: center

   Guanine

Our basis set is of the ANO-L type
contracted to C,N,O 4s3p1d / H 2s, plus 1s1p1d optimized
diffuse functions placed in the cation charge centroid.
The results are collected in :numref:`tab:gua1`.

.. table:: CASSCF and CASPT2 excitation energies (eV), oscillator strengths (f),
           dipole moments (:math:`\mu` (D)), and transition moment directions (:math:`\Theta`) of singlet valence excited states of guanine\ [#b]_. The Rydberg orbitals have not been included in the active space.
   :name: tab:gua1

   ============= ================ ================ ================ ================ ================ ================ ================ ===========================
   State         Theoretical                                                                          Experiment
   ------------- ------------------------------------------------------------------------------------ -------------------------------------------------------------
   |zws|         CAS              PT2              :math:`f`        :math:`\Theta`   :math:`\mu`      :math:`\Delta E` :math:`f`        :math:`\Theta`
   ============= ================ ================ ================ ================ ================ ================ ================ ===========================
   :math:`\pi`--:math:`\pi^*` transitions
   ----------------------------------------------------------------------------------------------------------------------------------------------------------------
   :math:`2^1A'` 5.72             4.47             0.20             |-|\64\ |o|      1.07             4.4--4.5         0.16             (|-|\4\ |o|,35\ |o|)
   :math:`3^1A'` 6.74             5.30             0.09             +52\ |o|         2.72             4.9--5.0         0.25             (|-|\75\ |o|)
   :math:`4^1A'` 7.18             5.63             0.05             |-|\90\ |o|      3.10             5.7--5.8         < 0.05
   :math:`5^1A'` 8.45             6.83             0.26             0\ |o|           3.20             6.1--6.3         0.41             (|-|\71\ |o|,\ |-|\79\ |o|)
   ============= ================ ================ ================ ================ ================ ================ ================ ===========================

.. [#b] See ref. :cite:`Fuelscher:97a` for details.

There are important discrepancies between theoretical and
experimental results, more important in the properties such as the
intensities and the transition dipole moments than in the excitation
energies. If we analyze the CASSCF output everything
is apparently correct: six converged roots, all of them clear valence
states, and no Rydberg orbital into the active space. This is the problem.
At least one of the Rydberg orbitals should have been introduced into the
active space. Rydberg and valence orbitals must be treated simultaneously
and this is not possible if there is no Rydberg orbital in the active space.

.. index::
   single: Orbitals; Deleted
   single: RASSCF; Delete
   single: Orbitals; Rydberg

The correct way to proceed is to take the first Rydberg orbital (3\ |pz|)
and place it as the 11th active orbital of :math:`a''` symmetry. Then
the CASSCF calculation will retain it in the space. Once the calculation
has converged we observe than at least one of the computed states is of
Rydberg character. It can also happen that some mixing appears in the
valence states due to the presence of the diffuse orbital in the active space.
The Rydberg orbital is then removed (placed in the last
position of its symmetry and the :kword:`DELEte` option used)
from the active space and the calculation
repeated. This time the next Rydberg orbital (3\ |dxz| or 3\ |dyz|) will take
its place. The process is repeated once again until the three Rydberg
orbitals have been first included in the active space and then
deleted (option :kword:`DELEted` of the RASSCF program). Now
we can reduce the active space to (0,10), only including valence orbitals
and valence excited states.

We can repeat the calculation including even more roots. The results
are in :numref:`tab:gua2`.

.. |bb| replace:: :math:`\Bigg\}`

.. table:: CASSCF and CASPT2 excitation energies (eV), oscillator strengths (f), dipole moments (:math:`\mu` (D)),
           and transition moment directions (:math:`\Theta`) of singlet valence excited states of guanine\ [#c]_ [#d]_. The Rydberg orbitals have been first included in the active space and then deleted.
   :name: tab:gua2

   +---------------+----------------------------------------------------------------------------------------------+------+-------------------------------------------------------------------+
   | State         | Theoretical                                                                                  |      | Experiment                                                        |
   +---------------+------------------+------------------+------------------+------------------+------------------+------+------------------+------------------+-----------------------------+
   |               | CAS              | PT2              | :math:`f`        | :math:`\Theta`   | :math:`\mu`      |      | :math:`\Delta E` | :math:`f`        | :math:`\Theta`              |
   +===============+==================+==================+==================+==================+==================+======+==================+==================+=============================+
   | :math:`\pi`--:math:`\pi^*` transitions                                                                                                                                                  |
   +---------------+------------------+------------------+------------------+------------------+------------------+------+------------------+------------------+-----------------------------+
   | :math:`2^1A'` | 6.08             | 4.76             | 0.133            | |-|\15\ |o|      | 7.72             |      | 4.4--4.5         | 0.16             | (|-|\4\ |o|,35\ |o|)        |
   +---------------+------------------+------------------+------------------+------------------+------------------+------+------------------+------------------+-----------------------------+
   | :math:`3^1A'` | 6.99             | 5.09             | 0.231            | +73\ |o|         | 6.03             |      | 4.9--5.0         | 0.25             | (|-|\75\ |o|)               |
   +---------------+------------------+------------------+------------------+------------------+------------------+------+------------------+------------------+-----------------------------+
   | :math:`4^1A'` | 7.89             | 5.96             | 0.023            | +7\ |o|          | 5.54             |      | 5.7--5.8         | < 0.05           |                             |
   +---------------+------------------+------------------+------------------+------------------+------------------+------+------------------+------------------+-----------------------------+
   | :math:`5^1A'` | 8.60             | 6.65             | 0.161            | |-|\80\ |o|      | 10.17            |      | 6.1--6.3         | 0.41             | (|-|\71\ |o|,\ |-|\79\ |o|) |
   +---------------+------------------+------------------+------------------+------------------+------------------+------+------------------+------------------+-----------------------------+
   | :math:`6^1A'` | 9.76             | 6.55             | 0.225            | |-|\41\ |o|      | 6.11             |      |                  |                  |                             |
   +---------------+------------------+------------------+------------------+------------------+------------------+      |                  |                  |                             |
   | :math:`7^1A'` | 8.69             | 6.66             | 0.479            | +43\ |o|         | 6.57             | |bb| | 6.6--6.7         | 0.48             | (|-|\9\ |o|,41\ |o|)        |
   +---------------+------------------+------------------+------------------+------------------+------------------+      |                  |                  |                             |
   | :math:`8^1A'` | 9.43             | 6.77             | 0.098            | +52\ |o|         | 7.17             |      |                  |                  |                             |
   +---------------+------------------+------------------+------------------+------------------+------------------+------+------------------+------------------+-----------------------------+

.. [#c] See ref. :cite:`Fuelscher:97a` for details.
.. [#d] A better match with the experimental values is obtained by considering solvent effects.

The results are quite different from those obtained previously, especially
regarding the
oscillator strengths and transition dipole moment directions. What we
have before was a set of states with valence--Rydberg character,
although it was not reflected in the orbital extension or population
analysis because the orbitals in the active space were too compact
to be able to reflect it. The states we have now are also of clear valence character
but the difference is that we have first included the Rydberg orbitals
in the active space, allowed the flexibility to describe the
Rydberg state, and then removed them from the space to finish with a
set of compact valence orbitals which cannot represent the Rydberg states.
Then, the latter are removed from the computed spectrum of states.

The experience of this type of treatment in different molecules
:cite:`Roos:96b,Roos:95b,Fuelscher:97a` points out that if the valence states
of a molecule are computed without considering the Rydberg states and
functions (whether by excluding them from the basis set or from the
active space) can result in an additional CASPT2 error as large as 0.3--0.4 eV.
The errors are
more severe for other transitions properties. One example of this can be
found for two different CASPT2 treatments of the formamide molecule,
one including diffuse functions and other excluding
them (see ref. :cite:`Serrano:96c` for details). Notice, however, that this
approach cannot describe a true valence--Rydberg mixing.
An alternative to such an approach is to use the Multi-State
CASPT2 treatment that, although computationally expensive, might properly
treats the valence--Rydberg mixing. It must be remembered, however, that
the performance of the MS-CASPT2 method relies on the previous mixing of
the wave functions, and therefore it will not be unusual, depending on the
employed basis set, to obtain CASPT2 results that already give the same
answer as MS-CASPT2 results when the initial basis sets are changed.

Other cases
-----------

The calculations become increasingly difficult with increased
size of the system or in low symmetry cases. Common
problems one has to solve are the selection of the active space when it
is not possible to include all orbitals expected to be important and
the presence of artificial valence-Rydberg mixing in the description
of the states. Specific problems appear in systems containing transition
metals, where there are a large amount of states close in energy.

.. index::
   single: Active space
   single: Program; RASSCF
   single: RASSCF

To include all the required orbitals into the active space is sometimes
impossible. This is one of the important limitations of the methodology.
But some solutions are
available if one is aware of the limitations. References
:cite:`Merchan:94b` and :cite:`Serrano:97a` report
studies on the porphin and indigo molecules, respectively.
Porphin and indigo have 24 and 20 :math:`\pi`,\ :math:`\pi^*` orbitals, respectively.
It is obviously impossible to include all of them in the active
spaces. The analysis of the configurations and occupation numbers
of the orbitals in a restricted number of excited states by means of
the RASSCF method has been found to be a useful procedure to
find a proper active space to study different states of the systems.
The RASSCF method is able to deal with a larger number of configurations
making possible to include all the :math:`\pi` orbitals in the active space
and analyze the role of the different orbitals. Our goal in this case
is to be able to discard some of the deepest or highest orbitals if they
become less important in the description of the desired states.

One possibility is to perform a SDTQ calculation involving all the
presumably important active space (occupied orbitals in :kword:`RAS1`,
empty orbitals in :kword:`RAS3`, no orbitals in :kword:`RAS2`, and four
holes/electrons allowed in RAS1/RAS3). The occupation numbers for the
active orbitals obtained for such calculation are usually similar to
those of a full CASSCF treatment. Another possibility is to place in
the CAS space (:kword:`RAS2`) the most important orbitals and
the corresponding electrons and only allow singles and doubles
excitations from :kword:`RAS1` (occupied orbitals) to
:kword:`RAS3` (empty orbitals). In all these cases we will study
the configurations and occupation numbers of the orbitals to find if
some of them are or minor importance for the description of the states
we are considering and then reduce the active space for the CASSCF/CASPT2
calculation :cite:`Merchan:94b,Serrano:97a`.

.. index::
   single: Transition metals; Active space
   single: CASPT2; g1 option

Calculation on the excited states of transition metal compounds have to
deal with another set of problems. For instance, the known 3d double-shell
effect: two sets of d orbitals (3d and 4d) must be included in the
reference space in order to obtain accurate results :cite:`Roos:96b` in
molecules containing metal atoms of the first transition row with many
d-electrons (:math:`\ce{Fe}`\--\ :math:`\ce{Zn}`). This
is a severe limitation when more ligands are included together with the
metal atom. Illustrations of such problems are the calculation of the
cyanide and carbonyl transition metal compounds :cite:`Roos:96b,Pierloot:93a`
and metal--protein models :cite:`Pierloot:96a`. Core--valence :cite:`Pierloot:93b`
and relativistic effects :cite:`Roos:96a` have been shown to be
important for obtaining accurate results. Finally, the problem of the high
multiplicity states in the standard CASPT2 formulation has to be considered.
The zeroth-order Hamiltonian is defined as a Fock-type one-electron operator.
Apart from the originally proposed Fock matrix :cite:`Andersson:90,Andersson:92a`,
a correction, denoted :math:`g_1` :cite:`Andersson:95a`, has been designed so that
CASSCF wave functions dominated by a closed-shell configuration, on the
one hand, and an open-shell configuration, on the other hand, are treated
in similar and balanced ways in the perturbation calculation. This
correction was shown to be essential in order to obtain reliable results
for the :math:`\ce{Cr_2}` molecule with the CASSCF/CASPT2 method :cite:`Roos:95b`.

.. index::
   single: Symmetry; Breaking

Each type of system and situation has its own specific problems. Size and
convergence problems in systems without any symmetry :cite:`Merchan:97,Serrano:95a`,
symmetry breaking and localization problems in high symmetry cases
:cite:`Merchan:96c`, excited states in radical cations :cite:`Fuelscher:95b` and
anions :cite:`Rubio:95c`, etc. In addition, there are situations such as
the crossing regions which require the simultaneous treatment of more than
one state at the CASPT2 level, which can only be solved using the multi-state
option in CASPT2.
