Electric Field: Homogeneous, Point Charges, Polarizability
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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.
