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 fi rst one is based on the use of the energy expressions supplied by the Theor y of Electronic Separability, as implemented in the ab initio perturbed ion (aiPI) method. The second one relies on the ionic electron densities gener ated by means of the aiPI model and the Electron Gas theory of Gordon and K im. In ionic solids, the many-body contributions mainly arise due to the change s of the ionic electron densities in passing from the gas-phase to the crys tal. 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 t he pair potentials. Finally, we compare the results of the different scheme s, showing that the inclusion of the self-energy dependence on crystal stra ins is crucial in the determination of sensible properties like phase trans ition pressures.