Ionic Models

Abstract This chapter presents models for interatomic potentials in ionic materials. As for the other models described in this book, the derivations are based on density functional theory and the variational principle, which leads to all the commonly used types of model. The simplest model of all is the rigid ion model, or Born model. It is introduced here by changing the charges on the nuclei in a rare gas solid and applying first-order perturbation theory. To go beyond the rigid ion model the second-order functional derived previously is invoked. This leads to a class of models with deformable ions, the simplest example of which has the same structure as the classical shell model. Compressible ion and variable charge transfer models are discussed. Classical charge transfer models are seen as equivalent to self-consistent tight-binding model in which the effect of covalency is neglected.