Electrostatic potential at an ionic crystal surface

A geometry for simulation of polar surfaces of ionic crystals is described. The simulation cells form a two-dimensional periodic array with one side bounded by the polar surface of a large piece of rigid ionic crystal. To simulate a polar surface in this way, the surface must carry excess charge to ensure that the cells are a model of the surface of a piece of crystal in bulk thermodynamic equilibrium. The electrostatic energy of the simulation cells is presented as a sum of convergent two dimensional lattice sums. The cancellation of divergences implicit in this energy because of the excess charge distributions is described in detail.