An ab initio embedded-cluster approach to electronic structure calculations on perfect solids: a Hartree-Fock study of lithium hydride

An ab inition embedded-cluster approach to electronic structure calculations on crystalline solids, within the framework of the linear combination of atomic orbitals method, is presented. The solid is modelled as a central cluster embedded in the field created by its environment (rest of the infinite solid). The environment field consisting of electron-nucleus and electron-electron interactions (both Coulombic and exchange) is constructed in a fully ab initio manner, and does not involve any model potential or any adjustable parameters. The electron-nucleus and Coulomb part of the electron-electron interactions are evaluated for the infinite solid using the Ewald summation technique without resorting to any multipole expansion. The exchange interaction is calculated over a finite range called the “short-range environment”. The orthogonality of the orbitals of the short-range environment to those of the central cluster is achieved by including projection operators in the Fock operator, which also has the effect of localizing these orbitals. Thus the method is expected to be useful for ab initio calculations on ionic and covalent crystals. Its applications to the lithium hydride crystal are presented.