Molecules with dipoles in periodic boundary conditions in a tetragonal cell.

When a system which contains a dipole, and whose dimensionality is
 less than three, is studied in a code which imposes periodic boundary
 conditions in all three dimensions, an artificial electric field
 arises which keeps the potential periodic. This has an impact on
 the total energy of the system, and on any other attribute which
 would respond to an electric field. Simple corrections are known for
 0D systems embedded in a cubic geometry, and 2D slab systems. This
 paper shows how the 0D result can be extended to tetragonal
 geometries, and that for a particular $c/a$ ratio the correction is zero. It
 also considers an exponential error term absent from the usual
 consideration of 2D slab geometries, and discusses an empirical form
 for this.