40.1.6 Options for numerical gradients

Numerical gradients can be computed with respect to variables on which the Z-matrix depends or with respect to Cartesian coordinates. In the latter case, it is most efficient to use symmetrical displacement coordinates. These do not change the symmetry of the molecule and the number of displacements is minimal. Alternatively (mainly for testing purpose) the gradients can be computed using symmetry unique Cartesian displacements or all 3N Cartesian displacements. In these cases the symmetry of the molecule can be reduced by the displacements and using such displacements is normally not recommended.

DISPLACE=ZMAT|SYMM|UNIQUE|CART

  
Displacement coordinates to be used for numerical gradient. The default is ZMAT if the geometry is given as a zmatrix which depends on variables, and SYMM (symmetrical displacement coordinates) otherwise. The use of UNIQUE or CART is not recommended.

SYMMETRY=AUTO|NOSYM

Symmetry to be used in wavefunction calculations of numerical gradients. This option is only relevant if DISPLACE=UNIQUE|CART. If AUTO is given, the maximum possible symmetry is used for each displacement. This implies that the energy is independent of the symmetry used. Note that this often not the case in MRCI or CASPT2 calculations. The option can also not be used in local correlation calculations.

AUTO

(logical). Same as SYMMETRY=AUTO

NOSYM

(logical). Same as SYMMETRY=NOSYM

RSTEP=rstep

Step length for distances in numerical gradient calculations (in bohr). The default is 0.01.

DSTEP=dstep

Step length for symmetrical displacements (in bohr). The default is 0.01.

ASTEP=astep

Step length for angles in numerical gradient calculations (in degree). The default is 1.

FOURPOINT

(logical). Use 4-point formula for accurate numerical gradient.

NUMERICAL

(logical). Force the use of numerical gradients, even if gradients are available.