Modified Fokker-Planck equations

A rotational FP-like equation is derived for the motion of a tagged diatomic molecule in a bath of diatomics possessing rotational and translational degrees of freedom. The torque correlation operator in the FP-like equation has contributions from (1) rotation-translation coupling; (2) collisions which change the orientation of the molecule as well as its momentum; and (3) non-linear momentum relaxation. An approximate improved FP collision operator is suggested and is employed in the calculation of the orientational distribution function and correlation time, τl , for a high density fluid in the strong collision regime. A translational FP equation for non-spherical molecules is also obtained by analogy with the rotational case.