MOLPRO

42 GEOMETRY OPTIMIZATION (OPTG) 315
42.2.10 Numerical gradients (NUMERICAL)
NUMERICAL,options,active 1 =step 1 , active 2 =step 2 . . . ;
With this directive the gradients are computed by finite differences. step i is the increment for
the active geometry parameter active i . For active parameters which are not specified, the default
values are used. By default, the increment is 0.01 bohr for bond distances and 0.5 or 1 degree
for angles less than or greater than 90 degrees, respectively. These defaults can be modified by
specifying RSTEP or ASTEP. DSTEP is the length of symmetrical displacements, which are
used if the optimization is performed in 3N coordinates.
For each active variable, two energy calculations are necessary in each geometry optimization
step – so numerical optimizations may be expensive! In optimizations of 3N coordinates
symmetrical displacement coordinates are normally used to minimize the number of energy
calculations. (see section 41.2.1).
For optimization of special energies see VARIABLE section 42.2.17.
The following options can be given:
RSTEP=rstep Step length for distances (in bohr). The default is 0.01.
ASTEP=astep
DSTEP=dstep
CENTRAL
FORWARD
FOURPOINT
Step length for angles (in degree). The default is 0.5 or 1 for angles
below and above 90 degree, respectively.
Step length for symmetrical displacements (in bohr). The default is
0.01.
Use central differences for gradient (default)
Use forward differences (not recommended for gradient).
Use four-point formula for very accurate numerical gradients.
PROCEDURE=procname Use given procedure for numerical calculation of the gradient. This
procedure must define a complete energy calculation (orbital optimization
and correlation treatment).
VARIABLE=varname
DISPLACE=type
Use given variable for numerical calculation of the gradient.
The displacement type. Note that the displacement type for gradient
and hessian must be the same. type can be one of the following:
SYMM
CART
UNIQUE
Use symmetric displacement coordinates (default). This
is the only recommended option.
Use 3N cartesian displacements (not recommended). This
requires many more energy calculations than necessary
and does not preserve the molecular symmetry.
Use symmetry-unique cartesian displacements (not recommended)
42.2.11 Transition state (saddle point) optimization (ROOT)
ROOT,root
Specifies the eigenvector of the hessian to be followed.
root=1
root=2
specifies a minimization (default).
specifies a transition state (saddle point) optimization.