A memory-function approach to the angular correlation functions in anisotropic liquids

A memory-function approach based on the Mori theory is used to test the validity of the extended diffusion equation commonly adopted to calculate angular correlation functions in partially oriented liquids. It is shown that the two methods give the same results, under the assumption of no correlation between angular variables and angular velocities, as far as the long-range terms of the intermolecular potential responsible for the orientational order are significantly smaller than the short-range terms which determine the magnitude of the friction tensor in the isotropic phase. This implies that the extended diffusion equation can give a reasonably accurate description of the rotational motion of the molecules in liquid crystals even at moderately high values of the orientational torque.