Nonequilibrium distribution functions for general rigid bodies in axially symmetric environments

A vector ρ 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 ρ 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 ρ 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