It is possible to apply a uniform external electric field given by a sawtooth waveform during the SCF calculation and the geometry optimization. For example, an electric field of 1.0 GV/m (10 V/m) is applied along the a-axis, in your input file specify the keyword 'scf.Electric.Field' as follows:
scf.Electric.Field 1.0 0.0 0.0 # default=0.0 0.0 0.0 (GV/m)The sign of electric field is taken as that applied to electrons. If the uniform external electric field is applied to a periodic bulk system without vacuum region, discontinuities of the potential are introduced, which may cause numerical instability. On the other hand, for molecular systems, the discontinuities are located in the vacuum region, indicating that numerical instability may not be induced.
As an illustration of the electric field, changes of total charge in a nitrobenzene molecule induced by the electric field are shown in Fig. 22. We can see that a large charge transfer takes place among oxygens in -NO, para-carbon atom, and para-hydrogen atom. The input file is Nitro_Benzene.dat in the directory 'work'. See also Section 'Analysis of difference in two Gaussian cube files' as for the difference charge maps shown in Fig. 22.