Studies of valence and core electron binding energies by a valence–electron molecular orbital theory based on model potentials

Abstract

A valence–electron theory is described in which core electrons are explicitly excluded, and in which their effects on valence electrons are incorporated into model potentials. The model potentials are deduced for atomic cores from atomic calculations, and the molecular core model potential is a superposition of the atomic ones. The basic atomic model potential consists of a core-charge coulomb potential modified by the addition of a short-range gaussian screened coulomb potential. This potential is particularly convenient to use with gaussian basis functions. Applications have been made within the LCAO SCF MO model to Li₂, LiH, LiF, (LiH)₂, Be, BeH₂, and BeO. Double zeta quality basis sets have been used, and parallel all-electron ab initio wavefunctions determined for comparison. For the Li systems valence orbital energies are in excellent agreement from both ab initio and the model potential MO. Similarly, shifts in the potential at the Li nuclei calculated from the model potential wavefunctions agree well with Koopmans' theorem Li 1s binding energy shifts from the ab initio wavefunctions. Results for the Be systems are relatively poor, and some explanatory speculation relevant to the 2s-2p near-degeneracy and the different nature of model potentials for the s and p orbitals is given.