Electrostatic potential from embedded clusters

We have calculated the electrostatic potential above the (100) surface of MgO using different cluster models, embedded in a representation of the surrounding lattice that takes into account quantum mechanical interactions as well as the full Madelung potential of the lattice. The use of embedded clusters results in electrostatic potentials, for both the Mg2+ and O2− sites, similar to those obtained by periodic calculations, independently of cluster size. The results are also similar for both stoichiometric and non-stoichiometric clusters. Relatively small clusters, if properly embedded, provide a good representation of the electrostatics above an ionic surface.