Pairwise-additive models for atom-surface interaction potentials: An ab initio study of He-LiF.

Abstract

Ab initio self-consistent-field calculations of the repulsive part of the He-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ and He-${\mathrm{Li}}^{+}$ interaction potentials are performed, together with coupled Hartree-Fock calculations of the dispersion coefficients determining the attractive part. The results are then used to construct an atom-surface potential for He-LiF, based on pairwise additivity of atom-ion forces. Ions in the crystal are simulated by performing calculations on a cluster consisting of an anion and its shell of nearest-neighbor cations, embedded in a point-charge lattice. Environments appropriate to ions in the bulk or at the surface can be constructed. Repulsion potentials between He atoms and in-crystal ions are found to be significantly weaker than those involving free ions. Anions at the surface are found to be slightly more polarizable then those in the bulk, but both are much less polarizable than free anions. Dispersion coefficients involving ions in different environments show similar trends. A model of the atom-surface potential is proposed, based on pairwise additivity but including corrections for induction energy, nonadditive dispersion forces, and dielectric screening effects. With semiempirical values of the dispersion coefficients based on the ab initio calculations, the resulting atom-surface potential has a well depth of 8.11 meV, compared with an experimental value of approximately 8.7 meV.