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
variables, the gradient is computed for all coordinates and
symmetrical displacement coordinates are used to evaluate the gradient. This
yields the minimum computational effort.
45.2.1 Choice of coordinates (COORD)
These defaults can be modified using the COORD directive:
where coord_type can be one of the following:
- Compute the numerical gradients for all variables on which the geometry depends (default).
- 3N or CART
- Compute the gradients for all 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 coordinates. It can also be given using
displacement_type can be one of the following:
- 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
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.
- As CART, but symmetry-equivalent displacements are
eliminated. Not recommended either.