Group theoretical analysis of lattice vibrations in molecular crystals

Computer programs have been developed by Warren and Worlton, for the external modes in ionic crystals where the principal axes of the radicals of groups coincide with the crystallographic axes. In this paper, we discuss the generalisation of these computer programs for molecular crystals where the principal axes do not coincide with the crystallographic axes and for molecular crystals with linear molecules having a redundant rotational degree of freedom. Our results are discussed for the q→O modes of solid NH₃, N₂ and CS₂.