Simulation of ionic crystals: The ab initio perturbed-ion method and application to alkali hydrides and halides.

Abstract

A new theoretical scheme appropriate for studying the electronic structure of ionic crystals is presented and applied to nine AB-type cubic lattices (A=Li,Na,K; B=H,F,Cl). The scheme, called the ab initio perturbed-ion (PI) method, is based on the theory of electronic separability and the ab initio model-potential approach of Huzinaga. In the PI method, the self-consistent-field (SCF) equations for each different lattice ion are first solved in a lattice potential that contains nuclear attraction, Coulombic, and nonlocal exchange operators, and lattice projectors enforcing the required ion-lattice orthogonality. The ionic SCF solutions are then used to compute the lattice potential, and the process is repeated until ion-lattice consistency is achieved. The lattice energy and other equilibrium properties are immediately obtained from the PI wave functions. The most remarkable ideas suggested by this crystal simulation are the following: (a) The crystal potential produces a contraction of the free-ion valence radial density, large for the anions but very small for the cations, that works as a bonding mechanism able to describe accurately the stability and equilibrium elastic constants of simple ionic crystals; (b) the PI method explains well the variation of several crystal properties with hydrostatic pressure; (c) the crystal bonding can be clearly analyzed in terms of simple cationic and anionic contributions. Moreover, the PI code may be used as an efficient source of environment-consistent ionic wave functions and energies.