Electric Field: Homogeneous, Point Charges, Polarizability

A homogeneous external electric field and/or the field due to point charges can be included in the Fock operator. Both can be applied in either a single-point calculation or in a geometry optimization. When applied in a geometry optimization, the molecule is allowed to rotate with respect to the point charges or to the electric field vector, but not to translate. Rigid translation is explicitly disabled to avoid drifting of the molecule in the external electric field.

Homogeneous electric fields, point charges, and external dipole moments should be defined in the System block, which is part of the AMS driver input. This section of the AMS driver manual describes the various input options.

Symmetry

The homogeneous electric field and the point charge fields may polarize the electronic charge density. This must be accounted for in the point group symmetry. If symmetry is not specified in the input and the homogeneous electric field and/or the point charges break symmetry, ADF uses symmetry NOSYM. You can specify a symmetry, but ADF will not check its correctness. An incorrect symmetry may lead to incorrect results.

Bonding energy

The bonding energy is computed as the energy of the molecule in the field minus the energy of the constituent fragments without the field. This may not always be the desired bonding energy. In the energy decomposition analysis (EDA), there is a separate term called ‘Electric Field (initial)’, which is the interaction energy of the electric field with the sum-of-fragments electron density and nuclei. You should subtract this term if you are interested in the energy of the molecule in the field minus the energy of the constituent fragments in the field.

Polarizability and hyperpolarizability

ADF supports a direct calculation of the (hyper)polarizability (see the section on Spectroscopic Properties). The static (hyper)polarizabilities can also be computed by applying a small homogeneous field and comparing the results with the field-free data.