Local Density Functional Theories of Ionic and Molecular Solids

Abstract

Publisher Summary This chapter presents local density functional theories of ionic and molecular solids. It describes a different application of density-functional theory, an approximate method called the (Gordon–Kim) electron-gas models that is useful for calculating interaction energies between closed-shell atoms and molecules. Fundamental to almost all discussions of molecular systems is the assumption that these systems consist of molecular units, with the electrons tightly bound to a specific molecule. Molecular and ionic solids, for instance, are characterized by having electrons highly localized about their parent nuclei, with very little electronic density in the interstitial regions. This extreme electron localization allows a great simplification in the application of local-density functional theories. To a first approximation, the electronic structure of interacting molecules can be taken as a sum of the unperturbed electron densities of those molecules. The evaluation of the energy is then simple; the electronic structure is given and the interaction energy alone needs to be evaluated. The chapter also discusses the problem of how to best approximate the electronic density of atoms and molecules in solids. It should be noted that in all the electron-gas-model calculations discussed, the total electron density is still represented as a sum of the distributions of the constituent molecules. However, instead of using gas-phase molecular charge distributions, one uses charge densities perturbed by the crystalline environment around the molecules.