MOLPRO

41 ENERGY GRADIENTS 303
ZMAT
Compute the numerical gradients for all variables on which the geometry depends
(default).
3N or CART Compute the gradients for all 3N nuclear coordinates. This is the default if the
z-matrix does not depend on variables or if the xyz input format is used. If this
option is used and the original geometry is given in z-matrix form, the z-matrix is
lost.
The specification of displacement type is optional and only affects the numerical calculation of
the gradient for 3N coordinates. It can also be given using
DISPLACE,displacement type
displacement type can be one of the following:
SYM
CART
Use symmetrical displacements. This yields the minimum number of displacements
and always preserves the symmetry of the wavefunction. This is the default
and only recommended option.
Displacements are generated for all 3N Cartesian coordinates. This is normally
not recommended, since in cases in which molecular symmetry is present it generates
far more displacements than needed. Also, the wavefunction symmetry is not
preserved, and the calculation must be done in C1 symmetry.
UNIQUE As CART, but symmetry-equivalent displacements are eliminated. Not recommended
either.
41.2.2 Numerical derivatives of a variable
Numerical derivatives of the value of a variable can be computed using
VARIABLE,name
The default is to compute the gradient of the current energy.
41.2.3 Step-sizes for numerical gradients
By default, the numerical step sizes are 0.01 bohr for distances or Cartesian coordinates, and 1
degree for angles. These defaults can be changed using
RSTEP,dr
ASTEP,da
where dr is the displacement for distances (or Cartesian coordinates) in bohr, and da is the
displacement for angles in degree. The value of RSTEP is used for symmetrical displacements.
The step sizes for individual variables can be modified using
VARSTEP,varname=value,. . .
where the value must be in atomic units for distances and in degree for angles.
41.2.4 Active and inactive coordinates
By default, numerical gradients are computed with respect to all variables on which the Z-matrix
depends, or for all 3N coordinates if there are no variables or XYZ inputstyle is used. One can
define subsets of active variables using