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Calculation With External Electric FieldsQuantum-chemical calculations for molecules in the presence of an external electrical field can be carried out by specifying the extrenal field via XFIELD= Nx These keywords (the corresponding default values are 0) request an electric field in X, Y, and/or Z direction with a strength of Nx*, Ny*, and/or Nz* 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 |