The Repulsive Potentials in Alkali Halide Molecules

On the basis of literature values of various spectroscopic quantities, the "experimental" five derivatives (1st to 5th) of the repulsive functions at the equilibrium distance were evaluated for the 20 alkali halide molecules, retaining the truncated Rittner model for the attractive forces. A self-consistency test showed that the used experimental values are reliable. Different analytical forms of the repulsive potential were then critically evaluated by comparison with the experimental derivatives. The repulsive functions were characterized by two, three, four or five empirical parameters. It has been shown that only functions with at least three parameters are sufficiently accurate to reproduce spectroscopic quantities such as ße and ye: the classical two parameter functions appeared too crude in this context.