Modified potential-induced-breathing model of potentials between close-shell ions

We have developed a simple ab initio model for the calculation of the thermoelastic properties of ionic compounds. The model is based on the Gordon-Kim-type electron-gas theory with spherically symmetric relaxation of ionic charge densities. The relaxation is controlled by a spherically averaged potential due to the total crystal charge density. The potential is self-consistent with the charge distribution, and contains Coulomb, exchange, and correlation contributions. We find that this potential yields anions that are slightly smaller than those stabilized by a point-ion Coulomb potential only. In the case of MgO, this results in a zero-pressure density that differs from experiment by less than 1%, a significant improvement over models that include only point-ion stabilization potentials. Further, the calculated equations of state and B1-B2 phase-transition pressures of NaCl, KCl, MgO, CaO, and SrO are in equally good agreement with data. The calculated equation of state and structure of the more covalent and less symmetric SiO₂ stishovite is only slightly less accurate.