Structure and energetics of alkali-metal halide dimer molecules

Abstract

The energetics and geometries have been computed for alkali-metal halide dimer molecules employing parameters for alkali halide diatomic molecules (monomers) obtained by fitting the potential functions to the experimental data on bond dissociation energies, equilibrium interionic distances, vibrational frequencies and dipole moments. The potential function used for this purpose is the ionic Rittner model as applied to the dimer molecules. It includes the coulombic, polarization, repulsion and dispersion energy terms. Contrary to previous work, the Rittner model gives results which are in good agreement with experiment and other calculations without altering the model or the input parameters. This success in duplicating the energetics and geometries of the dimers with a simple potential indicates that experimental origin and consistency are more important than the presumed physical significance of the potential. The anion–cation separations in the dimers are ca. 0.2–0.3 Å bigger than in the monomers and 0.2–0.3 Å smaller than in the corresponding alkali-metal halide crystals. For larger alkali-metal halides the overall potential is almost equal to the coulombic term as the other terms cancel each other out. The potential parameters derived from the scattering experiments and other sources are not successful in reproducing the dimer potentials and the related equilibrium geometries.