Translational-rotational Coupling in Strongly Anharmonic Molecular Crystals with Orientational Disorder

We have developed a new lattice dynam ics sheme for handling large amplitude llbrational motions or hindered rotations, en­ harmonic translational vibrations and translation-rotation coupling in molecular crystals. This formalism is based on expanding the intermolecular potential in the molecular displacements and including the cubic and higher terms, while retaining its exact anisotropy* This potential is first used to construct mean field states for the molecular translations and hindered rotations, and next to solve the equations of motion for the crystal in the random phase approximation (RPA), which takes Into account the correlations between the molecular motions as well as translation-rotation coupling. It Is Illustrated that this scheme gives very accurate results fo r the ordered ot and 7 phases o f solid nitrogen, while it also yields, for the first time, a quantum dynamical description o f the molecular m otions In the plastic 0 phase and a fairly accurate a —0 transition temperature.