The coupling between translational and rotational motions

The coupling between molecular reorientation and translational displacement is studied in a fluid of rough spheres. The correlation functions frequently encountered in thermal neutron scattering and laser light scattering are computed using a binary collision approximation. These are compared with the corresponding uncoupled results. In the diffusion limit coupled and uncoupled diffusion coefficients are found. The Hubbard relation is generalized. The maximum deviation between the coupled and uncoupled results occur for wavenumbers commonly found in thermal neutron scattering. The ratio of uncoupled and coupled correlation times displays regions where translation-rotation coupling is clearly important. In these regions there are important differences in the computed coupled and uncoupled correlation functions.