Quantum Chemical Interfaces
Introduction
This document describes the features common to the quantum
chemical interfaces in ChemShell.
All such interfaces conform the the ChemShell
general energy/gradient interface.
In simple terms this means that this
documentation does not describe a single command, but a family of
commands (gaussian.init, gaussian.energy,
gaussian.kill etc). These command will generally
be called implicitly by other, higher-level ChemShell commands such as
newopt and dynamics
in response to the theory= argument presented to those commands.
As described
elsewhere the gaussian family of
commands take a set of arguments define the structure to be
operated on (coords=, and the location for storage of the energies
and gradients (energy= and gradient=, etc.
for simplicity these options are not discussed here.
Each specific interface may have additional options that control
operation, or activate code-specific features - see the documentation
for the GAMESS-UK,
MNDO,
Gaussian,
Turbomole,
MOPAC interfaces.
Command Line Arguments
| Argument |
Argument type |
Mandatory |
Default |
To specify |
| basis= |
keyword |
no |
STO-3G |
Required basis set (internal library keyword) |
| basisspec= |
Tcl list |
no |
use basis= choice |
Required basis set (ChemShell library) |
| jobname= |
string |
no |
the module name |
name to use as root for file names |
| listing= |
string |
no |
the module name |
where to output the job listing (includes option to select stdout) |
| unique_listing= |
boolean |
no |
no |
whether to save each output to a separate file |
| hamiltonian= |
string |
no |
hf or mndo |
choice of QM hamiltonian, e.g. hf, lda, blyp, b3lyp, mp2, am1, mndo |
| charge= |
integer |
no |
0 |
Molecular Charge |
| mult= |
integer |
no |
1 |
Spin Multiplicity |
| scftype= |
keyword |
no |
rhf |
SCF type, e.g. rhf, uhf (Note that RHF/UHF includes RKS/UKS) |
| accuracy= |
keyword |
no |
medium |
General specification on accuracy of calculation |
| direct= |
boolean |
no |
no |
whether to select direct rather than disk-based methods |
| symmetry= |
boolean |
no |
no |
whether to use symmetry |
| maxcyc= |
integer |
no |
100 |
number of SCF cycles permitted |
| dispersion_correction= |
string |
no |
- |
Dispersion correction, see
note 4 |
Notes
- Internally, the QM code interfaces manage basis sets using the
get_basis
procedure. When the basisspec=
argument is used, the value of the argument is interpreted as
described in the Basis Library section.
- symmetry=off is chosen as the default as most codes have trouble
using the vectors from a previous geometry point if the symmetry changes.
It is not possible to guarantee that symmetry will not change during an
optimisation (or more particularly) an MD simulation so the current default
is needed for robust general operation. It can be overridden if the QM code
can deal with the symmetry change.
- At present the exact meaning of the accuracy parameter has not
been defined, and it is assumed that medium will select the default choices
for the code in question.
- Dispersion correction (van-der-Waals term) added to quantum chemical
calculations. The method follows S. Grimme, J. Comput. Chem. 27,
(2006), 1787. The only keyword to be provided is the name of the
functional. This defines an empiric pre-factor. Values for the functionals PBE,
BLYP, BP86, TPSS, and B3LYP are available. In all other cases, a default value
of 1 for the pre-factor will be used. Note that a dispersion correction will
always be used if dispersion_correction= is specified. To revert to
the default (no correction) remove the keyword.
|
