XTB(1)
======
:doctype: manpage

NAME
----

xtb - performs semiempirical quantummechanical calculations,
      for version 6.0 and newer

SYNOPSIS
--------

*xtb* ['OPTIONS'] 'FILE' ['OPTIONS']

DESCRIPTION
-----------

The `xtb(1)` program performs semiempirical quantummechanical calculations.
The underlying effective Hamiltonian is derived from density functional
tight binding (DFTB). This implementation of the xTB Hamiltonian is currently
compatible with the zeroth, first and second level parametrisation for
geometries, frequencies and non-covalent interactions (GFN)
as well as with the ionisation potential and
electron affinity (IPEA) parametrisation of the GFN1 Hamiltonian.
The generalized born (GB) model with solvent accessable surface area (SASA)
is also available in this version.
Ground state calculations for the simplified Tamm-Dancoff approximation (sTDA)
with the vTB model are currently not implemented.

GEOMETRY INPUT
~~~~~~~~~~~~~~

The wide variety of input formats for the geometry are supported by using
the mctc-lib. Supported formats are:

- Xmol/xyz files (xyz, log)
- Turbomole's coord, riper's periodic coord (tmol, coord)
- DFTB+ genFormat geometry inputs as cluster, supercell or fractional (gen)
- VASP's POSCAR/CONTCAR input files (vasp, poscar, contcar)
- Protein Database files, only single files (pdb)
- Connection table files, molfile (mol) and structure data format (sdf)
- Gaussian's external program input (ein)
- JSON input with `qcschema_molecule` or `qcschema_input` structure (json)
- FHI-AIMS' input files (geometry.in)
- Q-Chem molecule block inputs (qchem)

For a full list visit: https://grimme-lab.github.io/mctc-lib/page/index.html

`xtb(1)` reads additionally `.CHRG` and `.UHF` files if present.

INPUT SOURCES
-------------
`xtb(1)` gets its information from different sources. The one with highest
priority is the commandline with all allowed flags and arguments described below.
The secondary source is the `xcontrol(7)` system, which can in principle
use as many input files as wished. The `xcontrol(7)` system is the successor
of the set-block as present in version 5.8.2 and earlier. This implementation
of `xtb(1)` reads the `xcontrol(7)` from two of three possible sources,
the local xcontrol file or the 'FILE' used to specify the geometry
and the global configuration file found in the `XTBPATH`.

OPTIONS
-------

*-c, --chrg* 'INT':: 
    specify molecular charge as 'INT', overrides `.CHRG` file and `xcontrol` option

*-c, --chrg* 'INT:INT':: 
    specify charges for 'inner region:outer region' for `oniom` calculation, overrides `.CHRG` file and `xcontrol` option

*-u, --uhf* 'INT'::
    specify number of unpaired electrons as 'INT', overrides `.UHF` file and `xcontrol` option

*--gfn* 'INT'::
    specify parametrisation of GFN-xTB (default = 2)

*--gfnff, --gff* ::
    specify parametrisation of GFN-FF

*--tblite*::
    use tblite library as implementation for xTB

*--spinpol*::
    enables spin-polarization for xTB methods (tblite required)

*--oniom* 'METHOD' 'LIST'::
    use subtractive embedding via ONIOM method. 'METHOD' is given as `high:low`
    where `high` can be 'orca', 'turbomole', 'gfn2', 'gfn1', or 'gfnff' and
    `low` can be 'gfn2', 'gfn1', or 'gfnff'.
    The inner region is given as comma-separated indices directly in the commandline
    or in a file with each index on a separate line.

*--etemp* 'REAL'::
    electronic temperature (default = 300K)

*--esp* ::
    calculate electrostatic potential on VdW-grid

*--stm* ::
    calculate STM image

*-a, --acc* 'REAL'::
    accuracy for SCC calculation, lower is better (default = 1.0)

*--vparam* 'FILE'::
    Parameter file for xTB calculation

*--alpb* 'SOLVENT' ['STATE']::
    analytical linearized Poisson-Boltzmann (ALPB) model,
    available solvents are 'acetone', 'acetonitrile', 'aniline', 'benzaldehyde',
    'benzene', 'ch2cl2', 'chcl3', 'cs2', 'dioxane', 'dmf', 'dmso', 'ether',
    'ethylacetate', 'furane', 'hexandecane', 'hexane', 'methanol', 'nitromethane',
    'octanol', 'woctanol', 'phenol', 'toluene', 'thf', 'water'.
    The solvent input is not case-sensitive.
    The Gsolv reference state can be chosen as 'reference' or 'bar1M' (default).

*-g, --gbsa* 'SOLVENT' ['STATE']::
    generalized born (GB) model with solvent accessable surface (SASA) model,
    available solvents are 'acetone', 'acetonitrile', 'benzene' (only GFN1-xTB),
    'CH2Cl2', 'CHCl3', 'CS2', 'DMF' (only GFN2-xTB), 'DMSO', 'ether', 'H2O',
    'methanol', 'n-hexane' (only GFN2-xTB), 'THF' and 'toluene'.
    The solvent input is not case-sensitive.
    The Gsolv reference state can be chosen as 'reference' or 'bar1M' (default).

*--cma* ::
    shifts molecule to center of mass and transforms cartesian coordinates
    into the coordinate system of the principle axis (not affected by
    `isotopes'-file).

*--pop*::
    requests printout of Mulliken population analysis

*--molden*::
    requests printout of molden file

*--dipole*::
    requests dipole printout

*--wbo*::
    requests Wiberg bond order printout

*--lmo*::
    requests localization of orbitals

*--fod*::
    requests FOD calculation

RUNTYPS
~~~~~~~
NOTE: You can only select *one* runtyp, only the first runtyp will be used
      from the program, use implemented composite runtyps to perform several
      operations at once.

*--scc, --sp*::
    performs a single point calculation

*--vip*::
    performs calculation of ionisation potential.
    This needs the .param_ipea.xtb parameters
    and a GFN1 Hamiltonian.

*--vea*::
    performs calculation of electron affinity.
    This needs the .param_ipea.xtb parameters
    and a GFN1 Hamiltonian.

*--vipea*::
    performs calculation of electron affinity and ionisation potential.
    This needs the .param_ipea.xtb parameters
    and a GFN1 Hamiltonian.

*--vfukui*::
    performs calculation of Fukui indices.

*--vomega*::
    performs calculation of electrophilicity index.
    This needs the .param_ipea.xtb parameters
    and a GFN1 Hamiltonian.

*--grad*::
    performs a gradient calculation

*-o, --opt* ['LEVEL']::
    call `ancopt(3)` to perform a geometry optimization,
    levels from crude, sloppy, loose, normal (default), tight, verytight
    to extreme can be chosen

*--hess*::
    perform a numerical hessian calculation on input geometry

*--ohess* ['LEVEL']::
    perform a numerical hessian calculation on an `ancopt(3)` optimized
    geometry

*--bhess* ['LEVEL']::
    perform a biased numerical hessian calculation on an `ancopt(3)` optimized
    geometry

*--md*::
    molecular dynamics simulation on start geometry

*--metadyn* ['int']::
    meta dynamics simulation on start geometry, saving 'int' snapshots
    of the trajectory to bias the simulation

*--omd*::
    molecular dynamics simulation on `ancopt(3)` optimized geometry,
    a loose optimization level will be chosen

*--metaopt* ['LEVEL']::
    call `ancopt(3)` to perform a geometry optimization,
    then try to find other minimas by meta dynamics

*--path* ['FILE']::
    use meta dynamics to calculate a path from the input geometry
    to the given product structure

*--reactor*::
    experimental

*--modef* 'INT'::
    modefollowing algorithm. 'INT' specifies the mode that should be
    used for the modefollowing.
    

GENERAL
~~~~~~~
*-I, --input* 'FILE'::
     use 'FILE' as input source for `xcontrol(7)` instructions

*--namespace* 'STRING'::
     give this `xtb(1)` run a namespace. All files, even temporary
     ones, will be named according to 'STRING' (might not work everywhere).

*--[no]copy*::
     copies the `xcontrol` file at startup (default = true)

*--[no]restart*::
     restarts calculation from `xtbrestart` (default = true)

*-P, --parallel* 'INT'::
     number of parallel processes

*--define*::
     performs automatic check of input and terminate

*--json*::
     write xtbout.json file

*--citation*::
     print citation and terminate

*--license*::
     print license and terminate

*-v, --verbose*::
     be more verbose (not supported in every unit)

*-s, --silent*::
     clutter the screen less (not supported in every unit)

*--ceasefiles*::
     reduce the amount of output and files written

*--strict*::
     turns all warnings into hard errors

*-h, --help*::
     show help page

*--cut*::
      create inner region for `oniom` calculation without performing any calcultion

ENVIRONMENT VARIABLES
---------------------
`xtb(1)` accesses a path-like variable to determine the location of its
parameter files, you have to provide the `XTBPATH` variable in the same
syntax as the system `PATH` variable. If this variable is not set, `xtb(1)`
will try to generate the `XTBPATH` from the deprecated `XTBHOME` variable.
In case the `XTBHOME` variable is not set it will be generated from the
`HOME` variable. So in principle storing the parameter files in the users
home directory is suffient but might lead to come cluttering.

Since the `XTBHOME` variable is deprecated with version 6.0 and newer
`xtb(1)` will issue a warning if `XTBHOME` is not part of the `XTBPATH`
since the `XTBHOME` variable is not used in production runs.

LOCAL FILES
-----------

`xtb(1)` accesses a number of local files in the current working directory
and also writes some output in specific files. Note that not all input
and output files allow the *--namespace* option.

INPUT
~~~~~

*.CHRG*::
   molecular charge as 'int'

*.UHF*::
   Number of unpaired electrons as 'int'

*mdrestart*::
   contains restart information for MD, *--namespace* compatible.

*pcharge*::
   point charge input, format is 'real' 'real' 'real' 'real' ['int'].
   The first real is used as partial charge, the next three entries
   are the cartesian coordinates and the last is an optional atom type.
   Note that the point charge input is not affected by a CMA transformation.
   Also parallel Hessian calculations will fail due to I/O errors when using
   point charge embedding.

*xcontrol*::
   default input file in *--copy* mode, see `xcontrol(7)` for details,
   set by *--input*.

*xtbrestart*::
   contains restart information for SCC, *--namespace* compatible.

OUTPUT
~~~~~~

*charges*::
   contains Mulliken partial charges calculated in SCC

*wbo*::
   contains Wiberg bond order calculated in SCC, *--namespace* compatible.

*energy*::
   total energy in Turbomole format

*gradient*::
   geometry, energy and gradient in Turbomole format

*hessian*::
   contains the (not mass weighted) cartesian Hessian, *--namespace* compatible.

*xtbopt.xyz*, *xtbopt.coord*::
   optimized geometry in the same format as the input geometry.

*xtbhess.coord*::
   distorted geometry if imaginary frequency was found

*xtbopt.log*::
   contains all structures obtained in the geometry optimization
   with the respective energy in the comment line in a XMOL formatted
   trajectory

*xtbsiman.log*,*xtb.trj.'int'*::
   trajectories from MD

*scoord.'int'*::
   coordinate dump of MD

*fod.cub*::
   FOD on a cube-type grid

*spindensity.cub*::
   spindensity on a cube-type grid

*density.cub*::
   density on a cube-type grid

*molden.input*::
   MOs and occupation for visualisation and sTDA-xTB calculations

*pcgrad*::
   gradient of the point charges

*xtb_esp.cosmo*::
   ESP fake cosmo output

*xtb_esp_profile.dat*::
   ESP histogramm data

*vibspectrum*::
   Turbomole style vibrational spectrum data group

*g98.out*, *g98l.out*, *g98_canmode.out*, *g98_locmode.out*::
   g98 fake output with normal or local modes

*.tmpxtbmodef*::
   input for mode following

*coordprot.0*::
   protonated species

*xtblmoinfo*::
   centers of the localized molecular orbitals

*lmocent.coord*::
   centers of the localized molecular orbitals

*tmpxx*::
   number of recommended modes for mode following

*xtb_normalmodes*, *xtb_localmodes*::
   binary dump for mode following

TOUCH
~~~~~

*xtbmdok*::
   generated by successful MD

*.xtbok*::
   generated after each successful `xtb(1)` run

*.sccnotconverged*::
   generated after failed SCC with printlevel=2

//////////////////
NAMING CONVENTIONS
------------------
//////////////////

WARNINGS
--------
`xtb(1)` can generate the two types of warnings, the first warning section
is printed immediately after the normal banner at startup, summing up the
evaluation of all input sources (commandline, xcontrol, xtbrc). To check
this warnings exclusively before running an expensive calculation a
input check is implemented via the *--define* flag. Please, study this
warnings carefully!

After `xtb(1)` has evaluated the all input sources it immediately enters
the production mode. Severe errors will lead to an abnormal termination
which is signalled by the printout to STDERR and a non-zero return value
(usually 128). All non-fatal errors are summerized in the end of the calculation
in one block, right before the timing analysis.

To aid the user to fix the problems generating these warnings a brief
summary of each warning with its respective string representation in the
output will be shown here:

*ANCopt failed to converge the optimization*::
   geometry optimization has failed to converge in the given number
   optimization cycles. This is not neccessary a problem if only a
   small number of cycles was given for the optimization on purpose.
   All further calculations are done on the last geometry of the
   optimization.

*Hessian on incompletely optimized geometry!*::
   This warning will be issued twice, once before the Hessian,
   calculations starts (it would otherwise take some time before
   this this warning could be detected) and in the warning block
   in the end. The warning will be generated if the gradient norm
   on the given geometry is higher than a certain threshold.

EXIT STATUS
-----------
*0*::
   normal termination of `xtb(1)`

*128*::
   Failure (termination via error stop generates 128 as return value)

BUGS
----
please report all bugs with an example input, `--copy` dump of internal settings
and the used geometry, as well as the `--verbose` output to xtb@thch.uni-bonn.de

RESOURCES
---------
Main web site: http://grimme.uni-bonn.de/software/xtb

COPYING
-------
Copyright (C) 2017-2023 Stefan Grimme

xtb is free software: you can redistribute it and/or modify it under
the terms of the GNU Lesser General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

xtb is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License
along with xtb.  If not, see <https://www.gnu.org/licenses/>.
