Nonequilibrium distribution functions for general rigid bodies in axially symmetric environments

A vector rho is introduced in such a manner that the equilibrium rotational distribution function for a general rigid body has a simple quadratic form both from a body-fixed and space-fixed frame of reference. It is shown that when considering nonequilibrium distribution functions, representations employing the components of rho generalize more easily than those employing the components of the angular momentum or angular velocity, and lead to forms with greater accuracy. The behavior of rho and its relation to the angular momentum of the system is explored in some detail. Comparisons are made with distribution functions generated from molecular dynamics simulations of H2O+ drifting in a helium bath gas under the influence of a uniform electric field.