An ‘ ab initio ’ Gaussian orbital calculation of the (100) surface of crystalline lithium hydride

Abstract

An ab initio computation has been performed for the (100) face of a lithium hydride ionic crystal. The computation follows a similar computation performed earlier for an infinite crystal of lithium hydride. A simple wavefunction of a type proposed by A. A. Frost was used in which pairs of electrons are assigned to orbitals described by simple spherical Gaussian functions, the positions and sizes of which are allowed to float to minimize the energy. In the surface computation the nuclei and the Gaussian orbitals in the ultimate and penultimate layers were allowed to float freely, those in the remaining layers being disposed as in the infinite crystal. It was found that, in the surface layer, the hydrogen nuclei were farther from the fixed (third) layer than the lithium nuclei. Also, while the orbitals of the lithium ions remained centred at the same place as the nuclei, corresponding to zero polarization of the lithium ions, the hydride ions showed considerable polarization, the centres of the orbitals being displaced considerably from the nuclei. The changes in the penultimate layer are also discussed. An estimate was made of the surface energy for this face of lithium hydride and the value found was reasonable when compared with semi-empirical values for the (100) surfaces of alkali halide crystals.