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Subsections
4.16 ALASKA and SLAPAF: A Molecular Structure Optimization
One of the most powerful functions of ab initio calculations is geometry
predictions. The minimum energy structure of a molecule for a given method and
basis set is instructive especially when experiment is unable to determine the
actual geometry. MOLCAS performs a geometry optimization with analytical
gradients at the SCF or RASSCF level of calculation, and with numerical
gradients at the CASPT2 level.
In order to perform geometry optimization an input file must contain
a loop, which includes several calls: calculation of integrals (SEWARD),
calculation of energy (SCF, RASSCF, CASPT2), calculation of gradients
(ALASKA), and calculation of the new geometry (SLAPAF).
This is an example of such input
&GATEWAY
coord= file.xyz
basis= ANOSMB
>> EXPORT MOLCAS_MAXITER=25
>> Do While <<
&SEWARD
&SCF
&SLAPAF
>> EndDo <<
The initial coordinates will be taken from xyz file file.xyz, and the geometry
will be optimized at the SCF level in this case. After the wave function calculation,
calculation of gradients is required, although code ALASKA is automatically
called by MOLCAS. SLAPAF in this case required the calculation of an
energy minimum (no input). Other options are transition states (TS), minimum energy
paths (MEPsearch), etc
The loop will be terminated if the geometry
converges, or maximum number of iterations (MaxIter) will be reached (the
default value is 50).
There are several EMIL commands
(see sect )
, which can be
used to control geometry optimization. For example, it is possible to execute
some MOLCAS modules only once:
>> IF ( ITER = 1 )
* this part of the input will be executed only during the first iteration
>> ENDIF
Program SLAPAF is tailored to use analytical or numerical gradients produced
by ALASKA to relax the geometry of a molecule towards an energy
minimum (default, no input required then) or a transition state. The program is also used for
finding inter state crossings (ISC), conical intersections (CI),
to compute reaction paths, intrinsic reaction coordinate (IRC) paths, etc.
Examples as how to use the SLAPAF code is displayed in following section 4.15.
Keyword  Meaning

TS  Computing a transition state

FindTS  Computing a transition state with a constraint

MEPsearch  Computing a steepestdescent minimum reaction path

ITER  Number of iterations

INTErnal  Definition of the internal coordinates



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Up: 4. Program Based Tutorials
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