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# Calculation With External One-electron Perturbation

Arbitray external one-electron perturbations can be switched on with a strength of N*{$10**(-6)$} a.u. via the keyword

FINITE_PERTURBATION=N

%perturbation
label, iatom

in the ZMAT which is needed to specify what perturbation is requested. label is a 8-character string indicating the type of perturbation and iatom is a I3 integer which (if required) specifies the requested nuclei.

Currently available one-electron perturbations are:

in this case, the labels are XX, XY, XZ, YY, YZ, or ZZ and no nucleus has to be specified.

the labels are EFGXX, EFGXY, EFGXZ, EFGYY, EFGYZ, or EFGZZ and the atoms for which the efg perturbation is switched on needs to be given.

Recommendations

Note that the application of an external electric field might lower the molecular symmetry (e.g., the $C_{2v}$ symmetry of H$_2$O is reduced to $C_s$ when an electric field in $X$ or $Y$ direction is applied). As this lowering is not automatically accounted for, it is strongly recommended to run finite-field calculations with SYM=OFF.

Note that the molecule might be oriented differently depending on the actual symmetry used in the calculation.

Finite-field calculations can be performed with the electric field turned on before the HF-SCF step (default, DIFF_TYPE=RELAXED) or after the HF-SCF step (DIFF_TYPE=UNRELAXED). Both options (which refer to orbital-relaxed and orbital-unrelaxed calculations, respectively) are meaningful in the context of CC theory.

Finite-field calculations are available for the following type of calculations

a) all single-point energy calculations

b) all gradient and first-order property calculations

c) relativistic corrections to the energy

For all other type of calculations, it is not guaranteed that the electric-field perturbations are correctly considered.

Examples