.. index::
   single: Reaction path
   single: PES
   single: Transition state
   single: IRC
   single: Intrinsic reaction coordinate
   single: Optimization; Transition state

.. _TUT\:sec\:path:

Computing a reaction path
=========================

.. only:: html

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

Chemists are familiarized with the description of a chemical
reaction as a continuous motion on certain path of the
potential energy hypersurfaces connecting reactants with
products. Those are considered minima in the hypersurface
while an intermediate state known as the transition state
would be a saddle point of higher energy. The height of the
energy barrier separating reactants from products relates
to the overall rate of reaction, the positions of the
minima along the reaction coordinate give the equilibrium
geometries of the species, and the relative energies relate
to the thermodynamics of the process. All this is known
as transition state theory.

The process to study a chemical reaction starts by obtaining
proper geometries for reactants and products, follows by finding the
position of the transition state, and finishes by computing
as accurately as possible the relative energies relating the
position of the species. To perform geometry optimizations
searching for true minima in the potential energy surfaces (PES)
is by now a well-established procedure (see :numref:`TUT:sec:optim`).
An stationary point in the PES is characterized by having all the
first derivatives of the energy with respect to each one of the
independent coordinates equal to zero and the second derivatives
larger than zero. First-order saddle points, on the contrary, have their
second derivatives lower than zero for one coordinate,
that is, they are maxima along this coordinate. A
transition state is defined as a saddle point having only
one negative second derivative along the specific coordinate
known as the reaction coordinate. To simplify the treatment a special
set of coordinates known as normal coordinates is defined
in a way that the matrix of second derivatives is diagonal.
A transition state will have one negative value in the
diagonal of such a matrix.

Finally once the reactant, product and transition state geometries have been established one
could perform a Intrinsic Reaction Coordinate (IRC) analysis. This to find the energy profile
of the reaction and also to establish that the found transition state is connected to the
reactant and the product.

.. index::
   single: Transition state

Studying a reaction
-------------------

The localization of the transition state of a reaction is of importance
in both a qualitative and quantitative description of the reaction mechanism and
the thermodynamics of a reaction.
In the following example we will locate the
transition state of the proton transfer reaction between the two species
in :numref:`Figures %s <fig:job0>` and :numref:`%s <fig:job5>`.
The example selected here is chosen
to demonstrate the steps needed to find a transition state. For that sake we have
limited our model to the SCF level of theory.

.. figure:: job0.*
   :name: fig:job0
   :width: 50%
   :align: center

   Reactant

.. figure:: job5.*
   :name: fig:job5
   :width: 50%
   :align: center

   Product

Reactant and product
....................

.. compound::

  The first step is to establish the two species in equilibrium. These calculations
  would constitute standard geometry optimizations with the input for the reactant

  .. extractfile:: advanced/OPT.reactant.input

    >>> Do while <<<

    &Seward

    Basis set
    C.cc-pVDZ....
    C1            -1.9385577715        0.0976565175        0.4007212526
    C2            -2.4151209200       -0.0592579424        2.8519334864
    C3             0.7343463765        0.0088689871       -0.7477660837
    End of Basis
    Basis set
    H.cc-pVDZ....
    H1            -4.3244501026        0.0091320829        3.6086029352
    H2            -0.8591520071       -0.2642180524        4.1663142585
    H3            -3.4743702487        0.3026128386       -0.9501874771
    End of Basis
    Basis set
    O.cc-pVDZ....
    O1             0.7692102769        0.1847569555       -3.0700425345
    O2             2.4916838932       -0.2232135341        0.7607580753
    End of Basis
    End of input

    >>> IF ( ITER = 1 ) <<<
    &SCF
    Core
    Charge = -1
    >>> ENDIF <<<
    &SCF &End
    LUMORB
    Charge = -1

    &Slapaf
    Iterations = 20
    >>> EndDo <<<

  resulting in the following convergence pattern ::

                          Energy     Grad     Grad          Step           Estimated  Hessian Geom Hessian
    Iter     Energy       Change     Norm      Max  Element  Max  Element  Final Energy Index Update Update

     1   -265.09033194 0.00000000 0.091418 0.044965 nrc003 0.069275  nrc003 -265.09529138  0  RF(S)   None
     2   -265.09646330-0.00613136 0.020358 0.008890 nrc003 0.040393  nrc008 -265.09684474  0  RF(S)   BFGS
     3   -265.09693242-0.00046912 0.011611-0.005191 nrc001 0.079285  nrc016 -265.09709856  0  RF(S)   BFGS
     4   -265.09655626 0.00037616 0.020775-0.010792 nrc016-0.070551  nrc016 -265.09706324  0  RF(S)   BFGS
     5   -265.09706308-0.00050682 0.003309-0.001628 nrc003-0.010263  nrc017 -265.09707265  0  RF(S)   BFGS
     6   -265.09707056-0.00000747 0.000958-0.000450 nrc011 0.017307  nrc017 -265.09707924  0  RF(S)   BFGS
     7   -265.09706612 0.00000444 0.002451 0.001148 nrc003-0.011228  nrc018 -265.09706837  0  RF(S)   BFGS
     8   -265.09707550-0.00000938 0.000516 0.000220 nrc001-0.004017  nrc014 -265.09707591  0  RF(S)   BFGS
     9   -265.09707586-0.00000036 0.000286 0.000104 nrc001 0.002132  nrc017 -265.09707604  0  RF(S)   BFGS

  .. Note: contains a nbsp

  and for the product the input

  .. extractfile:: advanced/OPT.product.input

    >>> Do while <<<

    &Seward

    Basis set
    C.cc-pVDZ....
    C1            -2.0983667072        0.1000525724        0.5196668948
    C2            -2.1177298783       -0.0920244467        3.0450747772
    C3             0.5639781563        0.0024463770       -0.5245225314
    End of Basis
    Basis set
    H.cc-pVDZ....
    H1            -3.8870548756       -0.0558560582        4.1138131865
    H2            -0.4133953535       -0.2946498869        4.2050068095
    H3            -1.3495534119        0.3499572533       -3.3741881412
    End of Basis
    Basis set
    O.cc-pVDZ....
    O1             0.5100106099        0.2023808294       -3.0720173949
    O2             2.5859515474       -0.2102046338        0.4795705925
    End of Basis
    End of input

    >>> IF ( ITER = 1 ) <<<
    &SCF
    Core
    Charge = -1
    >>> ENDIF <<<
    &SCF
    LUMORB
    Charge = -1

    &Slapaf
    Iterations = 20
    >>> EndDo <<<

  resulting in the following convergence pattern ::

                          Energy     Grad     Grad          Step           Estimated  Hessian Geom Hessian
    Iter     Energy       Change     Norm      Max  Element  Max  Element  Final Energy Index Update Update

     1   -265.02789209 0.00000000 0.062885-0.035740 nrc006-0.060778  nrc006 -265.02939600  0  RF(S)   None
     2   -265.02988181-0.00198972 0.018235-0.011496 nrc006-0.023664  nrc006 -265.03004886  0  RF(S)   BFGS
     3   -265.03005329-0.00017148 0.001631-0.000978 nrc009-0.015100  nrc017 -265.03006082  0  RF(S)   BFGS
     4   -265.03004953 0.00000376 0.002464-0.000896 nrc014 0.013752  nrc017 -265.03006022  0  RF(S)   BFGS
     5   -265.03006818-0.00001865 0.001059 0.000453 nrc013-0.007550  nrc014 -265.03007064  0  RF(S)   BFGS
     6   -265.03006524 0.00000294 0.001800 0.000778 nrc014 0.006710  nrc014 -265.03007032  0  RF(S)   BFGS
     7   -265.03006989-0.00000465 0.000381 0.000190 nrc005 0.003078  nrc016 -265.03007014  0  RF(S)   BFGS
     8   -265.03006997-0.00000008 0.000129-0.000094 nrc016-0.001305  nrc017 -265.03007003  0  RF(S)   BFGS

.. Note: contains a nsbp

The computed reaction energy is estimated to about 42 kcal/mol at this level of theory.

Transition state optimization
.............................

To locate the transition state it is important to identify the reaction coordinate.
In our case here we note that the significant reaction coordinates are the bond distances between C1
and H3, and O1 and H3. In the location of the transition state we
will start from the geometry of the reactant for which the :math:`\ce{O{1}-H{3}}` bond distance is
2.51 Å. We will conduct the search in a number of constrained geometry
optimizations in which we step by step reduce the :math:`\ce{O{1}-H{3}}` distance towards the distance
in the product of 0.95 Å. The selected series is 2.0, 1.5, 1.3, and
1.0 Å.
To constraint the :math:`\ce{O{1}-H{3}}` bond distance we modify the input to the
:program:`GATEWAY` moduel by adding the following: ::

  Constraint
  R1 = Bond H3 O1
  Value
  R1 = 2.0 Angstrom
  End of Constraint

The :program:`SLAPAF` module's associated input looks like: ::

  &Slapaf &End
  Iterations
  20
  FindTS
  PRFC
  End of Input

This will correspond to the input for the first of the series of constraint
geometry optimization. However, note the keyword FindTS. This
keyword will make the SLAPAF module switch from a constrained geometry optimization
to a transition state geometry optimization if the updated geometrical
Hessian contains one negative eigenvalue. It is of course our hope that during the
series of constrained geometry optimizations that we will run into
this situation and find the transition state. The convergence pattern for the first
constrained optimization is ::

                        Energy     Grad     Grad          Step           Estimated  Hessian Geom Hessian
  Iter     Energy       Change     Norm      Max  Element  Max  Element  Final Energy Index Update Update

   1   -265.09707600 0.00000000 0.965614 0.965614 Cns001 0.230366* nrc009 -265.07671229  0  MFRFS   None
   2   -265.08759913 0.00947687 0.216939 0.214768 Cns001 0.081441  nrc012 -265.08946379  0  MFRFS   MSP
   3   -265.08218288 0.00541624 0.014770 0.007032 nrc010 0.019690  nrc010 -265.08242668  0  MFRFS   MSP
   4   -265.08251826-0.00033537 0.003644-0.001560 nrc003 0.005075  nrc002 -265.08254163  0  MFRFS   MSP
   5   -265.08254834-0.00003008 0.001274-0.000907 nrc012 0.026237! nrc016 -265.08257455  0  MFRFS   MSP
   6   -265.08251413 0.00003421 0.003036-0.002420 nrc016-0.024325  nrc016 -265.08254699  0  MFRFS   MSP
   7   -265.08254682-0.00003269 0.000837-0.000426 nrc012 0.012351  nrc017 -265.08255083  0  MFRFS   MSP
   8   -265.08255298-0.00000616 0.000470 0.000238 nrc016-0.005376  nrc017 -265.08255421  0  MFRFS   MSP
   9   -265.08255337-0.00000038 0.000329-0.000154 nrc012-0.004581  nrc014 -265.08255409  0  MFRFS   MSP
  10   -265.08255418-0.00000081 0.000206-0.000148 nrc012-0.000886  nrc014 -265.08255425  0  MFRFS   MSP
  11   -265.08255430-0.00000013 0.000123-0.000097 nrc012-0.001131  nrc014 -265.08255436  0  MFRFS   MSP

.. Note: contains a nbsp

Here we note that the Hessian index is zero, i.e. the optimization is a constrained
geometry optimization. The final structure is used as the starting geometry for
the 2nd constrained optimization at 1.5 Å. This optimization did not find a negative
eigenvalue either. However, starting the 3rd constrained optimization from the final
structure of the 2nd constrained optimization resulted in the convergence pattern ::

                        Energy     Grad     Grad          Step           Estimated  Hessian Geom Hessian
  Iter     Energy       Change     Norm      Max  Element  Max  Element  Final Energy Index Update Update

   1   -265.03250948 0.00000000 0.384120 0.377945 Cns001-0.209028* nrc007 -264.99837542 0  MFRFS   None
   2   -265.01103140 0.02147809 0.120709 0.116546 Cns001-0.135181  nrc007 -265.01209656 0  MFRFS   MSP
   3   -265.00341440 0.00761699 0.121043-0.055983 nrc005-0.212301* nrc007 -264.98788416 1  MFRFS   MSP
   4   -264.99451339 0.00890101 0.089986 0.045423 nrc007 0.123178* nrc002 -264.99582814 1  MFRFS   MSP
   5   -264.99707885-0.00256546 0.044095-0.015003 nrc009 0.159069* nrc015 -265.00090995 1  MFRFS   MSP
   6   -264.99892919-0.00185034 0.033489-0.013653 nrc015-0.124146  nrc015 -265.00050567 1  MFRFS   MSP
   7   -265.00031159-0.00138240 0.009416-0.004916 nrc018-0.156924  nrc018 -265.00070286 1  MFRFS   MSP
   8   -265.00019076 0.00012083 0.009057 0.005870 nrc018 0.081240  nrc018 -265.00049408 1  MFRFS   MSP
   9   -265.00049567-0.00030490 0.003380 0.001481 nrc011-0.070124  nrc015 -265.00056966 1  MFRFS   MSP
  10   -265.00030276 0.00019291 0.159266-0.159144 Cns001 0.114927! nrc015 -264.99874954 0  MFRFS   MSP
  11   -265.00098377-0.00068101 0.031621-0.008700 nrc005-0.101187  nrc007 -265.00046906 1  MFRFS   MSP
  12   -265.00050857 0.00047520 0.003360 0.001719 nrc015 0.012580  nrc015 -265.00052069 1  MFRFS   MSP
  13   -265.00052089-0.00001233 0.001243-0.000590 nrc017-0.006069  nrc017 -265.00052323 1  MFRFS   MSP
  14   -265.00052429-0.00000340 0.000753 0.000259 nrc011-0.002449  nrc018 -265.00052458 1  MFRFS   MSP
  15   -265.00052441-0.00000011 0.000442-0.000136 nrc007 0.003334  nrc018 -265.00052464 1  MFRFS   MSP
  16   -265.00052435 0.00000006 0.000397 0.000145 nrc017 0.001628  nrc010 -265.00052459 1  MFRFS   MSP

.. Note: contains a nbsp

Here a negative Hessian eigenvalue was found at iteration 3. At this point the optimization turn to a normal
quasi-Newton--Raphson optimization without any constraints. We note that the procedure flips back to a constrained
optimization at iteration 10 but is finished as an optimization for a transition state.
The predicted activation energy is estimated to 60.6 kcal/mol (excluding vibrational corrections).
The computed transition state
is depicted in :numref:`fig:job3`.

.. figure:: job3.*
   :name: fig:job3
   :width: 50%
   :align: center

   Transition state

The remaining issue is if this is a true transition state. This issue can only be
resolved by doing a calculation of the analytical Hessian using the
MCKINLEY module (execution of the MCLR module is automatic). The corresponding input is

.. extractfile:: advanced/MCKINLEY.input

  &Seward

  Basis set
  C.cc-pVDZ....
  C1            -1.8937541206        0.0797525492        0.5330826031
  C2            -2.3239194706       -0.0748842444        3.0012862573
  C3             0.7556108398       -0.0065134659       -0.5801137465
  End of Basis
  Basis set
  H.cc-pVDZ....
  H1            -4.2196708766       -0.0106202053        3.8051971560
  H2            -0.7745261239       -0.2775291936        4.3506967746
  H3            -1.9256618348        0.2927045555       -2.1370156139
  End of Basis
  Basis set
  O.cc-pVDZ....
  O1             0.2162486684        0.2196587542       -2.9675781183
  O2             2.8171388123       -0.2187115071        0.3719375423
  End of Basis
  End of input

  &SCF
  Charge = -1

  &McKinley
  Perturbation
  Hessian

.. compound::

  From the output of the MCLR code ::

          ***********************************
          *                                 *
          * Harmonic frequencies in cm-1    *
          * Intensities in km/mole          *
          *                                 *
          * No correction due to curvlinear *
          * representations has been done   *
          *                                 *
          ***********************************

        Symmetry a
       ==============

                             1         2         3         4         5         6

                Freq.   i2027.40     i2.00     i0.07      0.05      0.07      2.02

    ...
                             7         8         9        10        11        12

                Freq.       3.57    145.36    278.41    574.44    675.27    759.94

    ...
                            13        14        15        16        17        18

                Freq.     927.78    943.60   1000.07   1225.34   1265.63   1442.57

    ...
                           19        20        21        22        23        24

                Freq.    1517.91   1800.86   1878.11   2294.83   3198.94   3262.66

  we can conclude that we have one imaginary eigenvalue (modes 2--7 corresponds to the translational
  and rotational zero frequency modes) and that the structure found with this procedure indeed is a
  transition state. A post calculation analysis of the vibrational modes using the MOLDEN package
  confirm that the vibrational mode with the imaginary frequency is a mode which moves the proton from
  the oxygen to the carbon.

Finding the reaction path -- an IRC study
-----------------------------------------

A minimum energy path (MEP) is defined as the path defined by a sequence of geometries obtained by a
series of optimizations on a hypersphere. The series of constrained optimization starts from some
starting structure and the optimized structure at each step is taken as the start for the next step.
The constraint in these optimizations is the radius (in mass weighted coordinates) of the hyper sphere
with the origin defined by the starting geometry. If the starting structure is a transition state the
path is called an Intrinsic Reaction Coordinate (IRC) path. Since the transition structure (TS) has a negative
index of the Hessian we have two paths away from the TS. One leading us to the product(s) and one going to
the reactant(s). The IRC analysis is used to verify whether the TS is really connecting the expected
reactant(s) and product(s) and it is performed in forward and backward directions starting from the TS.
This analysis is obtained using the keyword :kword:`IRC` with the :program:`SLAPAF`
specifying the number of points and, if different from the default value (0.10 au), the radius
of the hypersphere with the keywords :kword:`nIRC` and :kword:`IRCStep`, respectively.
The reaction vector can be found on RUNOLD or it can be specified explicitly (see keyword :kword:`REACtion vector`).
In the latter case, the vector can be find at the end of the optimization job in the
``The Cartesian Reaction vector`` section of the :program:`SLAPAF` output.
A file named :file:`$Project.irc.molden` (read by :program:`MOLDEN`) will be generated
in $WorkDir containing only those points belonging to the IRC.
Here an example for an IRC analysis with 20 points back and forth and with 0.05 au as step.
The reaction vector will be read on RUNOLD. ::

  >>> EXPORT MOLCAS_MAXITER=500
  >>> Do while <<<

  ...

  &Slapaf &End
  IRC
  nIRC
   20
  IRCStep
   0.05
  Iterations
  200
  End of Input
  >>> EndDo <<<

If the file :file:`RUNFILE` is not available, the reaction vector must be specified in the
input. ::

  >>> EXPORT MOLCAS_MAXITER=500
  >>> Do while <<<

  ...

  &Slapaf &End
  IRC
  nIRC
   20
  IRCStep
   0.05
   REACtion vector
     0.140262        0.000000        0.179838
     0.321829        0.000000       -0.375102
    -0.006582        0.000000       -0.048402
    -0.032042       -0.018981       -0.003859
    -0.423466        0.000000        0.247525
  Iterations
  200
  End of Input
  >>> EndDo <<<
