[Wien] Electric Field Effects

[Paul Fons]

Hi All,
	I have been studying the work of Stahn, Pietsch, Blaha, and Schwartz on electric field induced charge density variations (in GaAs) [PRB v63, 165205 (2001).  I had a couple of quick questions.  In the paper a large (22 formula units) of GaAs is used and the field is inverted halfway across the cell in the z-direction (e.g. to maintain the periodic boundary conditions).  I am curious to know if such a large number of repeats is necessary or is it simply necessary to have the field applied over a minimum spatial length.  First, what is the case for GaAs, can the number of repeats be varied without large changes in the result?  The second question has more to do with the sort of systems I wish to calculate E-field-induced effects upon.  They are already quite large in size with about 20 layers in a supercell and I would like to avoid making the cell unnecessarily large.  Would it make sense to make a vacuum layer between two units cells (and leave the electrostatic inflection point there).  I assume I would have to fix the positions of the atoms at the end of the cell next to the vacuum layer if I did this.  Does this make sense?  Any advice on what cell sizes would be appropriate would be gratefully received.

				Paul Fons