MOLPRO

41 ENERGY GRADIENTS 302
ZMAT
OPT3N
RSTEP=rstep
DSTEP=dstep
ASTEP=astep
CENTRAL
FORWARD
FOURPOINT
NUMERICAL
VARSAV
(logical). Same as COORD=ZMAT
(logical). Same as COORD=3N
Step length for distances in numerical gradient calculations (in bohr).
The default is 0.01.
Step length for symmetrical displacements (in bohr). The default is
0.01.
Step length for angles in numerical gradient calculations (in degree).
The default is 1.
(logical). Use 2-point central formula; needs 2M energy calculations
for M degrees of freedom.
(logical). Use forward gradients (needs only M + 1 energy calculations,
but less accurate)
(logical). Use 4-point formula for accurate numerical gradient; needs
4M energy calculations.
(logical). Force the use of numerical gradients, even if gradients are
available.
(logical). Save gradient in variables GRADX, GRADY, GRADZ.
Example
hf
ccsd(t)
forces,numerical
The program will then automatically repeat HF and CCSD(T) at as many geometries as needed
for evaluating the gradient. This is equivalent to
hf
ccsd(t)
forces,numerical,startcmd=hf
or, using a procedure
forces,numerical,proc=runccsdt
...
runccsdt={
hf
ccsd(t)}
41.2.1 Choice of coordinates (COORD)
By default, the numerical gradients are computed relative to all variables on which the z-matrix
depends. If the z-matrix depends on no variables or on 3N variables, the gradient is computed for
all 3N coordinates and symmetrical displacement coordinates are used to evaluate the gradient.
This yields the minimum computational effort.
These defaults can be modified using the COORD directive:
COORD,coord type,[displacement type]
where coord type can be one of the following: