Dynamics of translations and rotations in molecular crystals

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Abstract
The dynamics of rotations and translations in molecular crystals is derived from a microscopic Hamiltonian. The dynamical variables are the lattice displacements and their conjugate momenta for the translations and, in the case of orientationally disordered phases, the appropriate symmetry‐adapted functions for the rotations. Coupled dynamical equations for the oscillatory lattice displacements and the orientational molecular relaxation are obtained. The coupling between these equations is expressed in terms of static susceptibilities which are calculated from the Hamiltonian. The coherent inelastic neutron scattering law, including interference between translations and rotations, is derived. Our results are relevant for inelastic neutron,Brillouin, and Raman scattering experiments. As a concrete example we refer to KCN.