Perturbation theory for orientational time-correlation functions

A simple perturbation theory is proposed for single molecule orientational time correlation functions Cl,m (t). These functions are obtained by approximating the corresponding memory functions Kl,m (t) by the product of the free rotor memory function times a gaussian decay involving the mean square torque on the reorienting axis in the molecule. After a brief discussion of some of the general features of this model, its predictions are compared with some computer simulations of liquid Br2. It is argued that this perturbation approach is reasonably accurate up to fairly large values of the mean square torque, but breaks down when the intermolecular potential and the fluid density and temperature are such as to produce significant librational character in the reorientation.