$EFIELD

$EFIELD group (not required)

This group permits the study of the influence of an external electric field on the molecule. The method is general, and so works for all ab initio SCFTYPs.

EVEC
= an array of the three x,y,z components of the applied electric field.

SYM
= a flag to specify when the field to be applied breaks the molecular symmetry. Since most fields break symmetry, the default is .FALSE.


Restrictions: analytic hessians are not available, but numerical hessians are. Because an external field causes a molecule with a dipole to experience a torque, geometry optimizations must be done in Cartesian coordinates only. Internal coordinates eliminate the rotational degrees of freedom, which are no longer free.

Notes: a hessian calculation will have two rotational modes with non-zero "frequency", caused by the torque. A gas phase molecule will rotate so that the dipole moment is anti-parallel to the applied field. To carry out this rotation during geometry optimization will take many steps, and you can help save much time by inputting a field opposite the molecular dipole. There is also a stationary point at higher energy with the dipole parallel to the field, which will have two imaginary frequencies in the hessian. Careful, these will appear as the first two modes in a hessian run, but will not have the i for imaginary included on the printout since they are rotational modes.


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