A point-charge representation of frost-model wave functions

A point-charge representation of Frost-model wave functions is derived from a symmetry-adapted perturbation theoretic expansion. The new point-charge model is simpler than those suggested previously yet gives good estimates of first-order molecular properties. The treatment can easily be extended to deal with second-order properties and, when this is done, formulae similar to those of the Drude theory are obtained. Using these formulae, theoretical expressions for the refractive indices of methane, ethane and water are computed and are in reasonable accord with experiment for the two hydrocarbons but less satisfactory for water.