A simple extension of the current models of lattice dynamics is used to evaluate the binding energy, the internuclear distance, the electric dipole, and the vibrational frequency of the alkali-halide molecules from solid-state properties. An extensive comparison with experimental data suggests that the models may be useful as an empirical scheme of interpolation between the perfect crystal and the molecule. A polarization catastrophe in the standard ionic theory is discussed.

• Received 20 February 1967

DOI:https://doi.org/10.1103/PhysRev.160.716