A three-variable theory on the translation-reorientation coupling of molecular liquids

The two-variable theory on the translation-reorientation coupling observed in the light-scattering spectra of molecular liquids is extended to include the time derivative of the orientation as one of the explicit variables. The novel “three-variable theory” has the mathematical form more proper than the two-variable one in that the shear viscosity is directly proportional to the time integration of the time-correlation function of the random shear stress and that the translation-reorientation coupling constant is given by the covariance of the random shear stress and random acceleration of the orientation. Molecular dynamics simulations on the isotropic phase of Gay-Berne fluid are performed to test the performance of the three-variable theory. The decay times of all the memory functions are much shorter than the corresponding ones of the two-variable theory, and they are rather insensitive to the density of fluid even near the isotropic-nematic transition where reorientational correlation time is long. These characteristics demonstrate that the three-variable theory provides a better starting point to discuss the translation-reorientation coupling based on the molecular interaction and liquids structure.