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# Calculation With External Electric Fields

Quantum-chemical calculations for molecules in the presence of an external electrical field can be carried out by specifying the extrenal field via

XFIELD= Nx
YFIELD = Ny
ZFIELD = Nz

These keywords (the corresponding default values are 0) request an electric field in X, Y, and/or Z direction with a strength of Nx*{$10**(-6)$}, Ny*{$10**(-6)$}, and/or Nz*{$10**(-6)$} a.u., respectively.

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

d) polarizability

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

Examples