Cumulant expansion of the orientational auto-correlation function

A general method is developed to relate the orientational and angular momentum auto-correlation functions of a vector embedded in the rotating asymmetric top. It is shown that a previous attempt along these lines by Nee and Zwanzig contains an error which is rectified in this paper. The reduction to the free rotor limit is discussed carefully. The continuity equation, whose solution is a time-ordered exponential does not produce an orientational a.c.f. valid at the free rotor limit. This is because in this limit the kinematic relation between orientation and angular velocity is no longer a multiplicative stochastic process.