Strategies for determining and using ab initio interionic potentials

In this contribution, we discuss two techniques for determining interionic potentials from quantum-mechanical descriptions of ions in crystals. The first one is based on the use of the energy expressions supplied by the Theory of Electronic Separability, as implemented in the ab initio perturbed ion (aiPI) method. The second one relies on the ionic electron densities generated by means of the aiPI model and the Electron Gas theory of Gordon and Kim. In ionic solids, the many-body contributions mainly arise due to the changes of the ionic electron densities in passing from the gas-phase to the crystal. We present several schemes to deal with these self-energy terms, both as a separate contribution to the total energy and as an implicit term in the pair potentials. Finally, we compare the results of the different schemes, showing that the inclusion of the self-energy dependence on crystal strains is crucial in the determination of sensible properties like phase transition pressures.