Lattice vibrations in crystals with deformable molecules; a calculation for naphthalene

Abstract

A calculation has been done to show the effect of molecular distortion on the external modes of the molecular crystal naphthalene. This calculation also yields the dispersion relation for the internal modes caused by the crystalline forces. The method used is quite general. The force field for the isolated molecule comes from spectroscopic frequency fitting, and to this is added the effect of crystalline forces using a Buckingham atom-atom potential. The shift and splitting of all the modes calculated at zero wave vector are given, and also the dispersion relation for the lower modes for wave vectors parallel to the crystal screw-diad axis. A suggestion is made concerning the analysis of the results of neutron coherent inelastic scattering, which until now has assumed molecular rigidity.