COULOMB ENERGY OF POLAR CRYSTALS

A simple derivation is given to arrive at an expression for the Coulom b energy of a polar crystal with emphasis on its dependence on the ext ernal form of the crystal. It is found that the existing treatments ar e incomplete, since they refer to the central cell in the crystal, whi ch may have an energy different from the average one. It is stressed t hat there is a very simple relation between results from calculations using a cutoff scheme and results from Ewald summation. Explicit formu las for the form-dependent contribution are developed for the case of a rectangular crystal. However, the usual approach in which that contr ibution is neglected appears to be a reasonable one for practical appl ications.