AB-INITIO PAIR POTENTIALS FROM QUANTUM-MECHANICAL ATOMS-IN-CRYSTALS CALCULATIONS

A new technique for deriving pairwise potentials from ab initio quantum-mechanical calculations of atoms in crystals is presented. The total energy of the crystal referred to the infinitely separated atoms is partitioned into two components: (i) a monocentric deformation energy arising from the changes of atomic electron density in passing from the free atom to the crystal state, and (ii) a bicentric energy due to the atomic interactions in the crystal. We show that the first component can be meaningfully separated into pairwise contributions. The new technique is used to derive Buckingham-type potentials for the alkali chloride crystals that (a) reproduce the ab initio crystal energy, (b) predict good static equations of state and defect properties, and (c) give a realistic description of the crystal binding.