Dynamic multipoles in molecular crystals

Abstract

The lattice energy of molecular crystals is expressed in terms of interactions between effective dynamical multipoles. The multipole moments are characteristic for a given molecule in a crystal; they depend on the arrangement of the molecules in the lattice and can be considered as self‐consistent ones. The cohesive energy of the crystal is described as interactions between static multipoles, i.e., those parts of the multipoles which do not depend on displacements of molecules treated as rigid bodies. The remaining parts of the moments, depending on molecular displacements, represent the multipole moments induced by the lattice vibrations. The general equation for lattice vibrations in molecular crystals is derived and the dynamical matrix is expressed by the effective charge distributions and lattice sums. The conception of dynamical multipoles put forward in this paper relates frequencies and infrared intensities of lattice vibrations within the same model.