Dynamics of translations and rotations in molecular crystals: Macroscopic and microscopic approaches

Starting from a Hamiltonian for an orientationally disordered crystal with bilinear coupling between translational and orientational coordinates, a nonequilibrium macroscopic free energy is calculated. It is used to obtain coupled dynamic equations for translations and rotations by methods of irreversible thermodynamics. The corresponding dynamic response functions are in full agreement with those derived using a microscopic Mori-type projection-operator method. The coefficients of the macroscopic free energy are related to the static susceptibilities obtained from the Hamiltonian in the framework of molecular-field theory. Dynamic sum rules for translational-rotational motion are given. © 1981 The American Physical Society.