Starting from a repulsive overlap potential, we derive a microscopic Hamiltonian that takes into account single particle orientational potential effects and rotational–translational coupling in molecular crystals. This coupling leads to an effective orientational interaction between nonspherical molecules. Theoretical expressions for the elastic constants in the presence of this interaction are derived. The anomalous thermoelastic behavior which was discovered by Haussühl for c₄₄ and c₁₁ in KCN and NaCN, and found by Rowe et al. in neutron measurements of TA phonons, is explained. The qualitative and quantitative agreement between theory and experiment is convincing.

We have derived a microscopic Hamiltonian that takes into account rotational-translational coupling in molecular crystals. This coupling leads to an effective orientational interaction among nonspherical molecules. It is responsible for an anomalous behavior of the elastic constants and of the transverse-acoustic phonons. The temperature behavior of $c_{44}$ and $c_{11}$ in KCN above the 168-K phase transition is explained.

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.

A generalised notion of exponential families is introduced. It is based on the variational principle, borrowed from statistical physics. It is shown that inequivalent generalised entropy functions lead to distinct generalised exponential families. The well-known result that the inequality of Cram´er and Rao becomes an equality in the case of an exponential family can be generalised. However, this requires the introduction of escort probabilities.