Vibrational and transport properties of molecular solids

In the present article we will study the following problems. We start with a formulation of molecular interactions in terms of symmetry adapted functions, taking into account the symmetry of the molecule and of the crystal site (Sect. 2). Next (Sect. 3) we study in detail the bilinear translation-rotation coupling and its relation to molecular symmetry. As particular examples we consider KCN and NaNO2. Using concepts from the theory of quantum crystal we propose in Sect. IV a renormalization of various coupling coefficients. Such a description is important near phase transitions where one observes a peculiar temperature dependence of interaction potentials. We treat the ferroelastic transition in KCN and the incommensurate transition in NaNO2. The following section V is devoted to dynamic equations for translation-rotation coupled systems and to a discussion of the corresponding resonances. Finally in Sect. VI we consider frequency dependent transport coefficients related to orientational relaxation and phonon viscosity