Lattice dynamics of molecular crystals using a molecular force field and an intermolecular potential function with application to the atom-atom model

Abstract

A hybrid method is suggested for the lattice dynamics of molecular crystals: a functional form such as the atom-atom model is assumed for the intermolecular part of the potential energy and force constants calculated from this directly; on the other hand, the intramolecular contribution is expressed in terms of parametric force constants which are chosen to fit molecular frequencies. The significance of the equilibrium conditions in these circumstances is carefully considered and terms are recognized which have not always been taken into account previously. The relationship of this method to the rigid molecule approximation is also discussed.