Rotational diffusion with memory

One of the virtues of using the G.L.E. to derive such (relatively) simple results is that one can go back and improve on the original model by refining the approximations made. For example, "rotational diffusion with memory" has been proposed by Nee and Zwanzig (13), who noted that one need not assume that the memory functions decay instantaneously on the time scale of the reorientation. They retain the approximation that all the time dependence comes from changes in the angular velocity, but suggested that the constants should be replaced by memory... 

The similarity exhibited here between the multi-variable and the memory hierarchy formulations of the orientational problem is obviously not a general result in fact, it derives from the choice of a variable and its time derivative in the multi-variable theory. Other variations on these themes can readily be devised for example, one could couple Q, and 4 (orthogonal ized to Qj, however) (8) or one could combine higher-order memory functions with a multi-variable theory or one could couple translational and rotational variables (9), The enormous flexibility of the G,L,E, means that the intuition of the user will play a particularly significant role in determining the success of the outcome. To illustrate this point, we now briefly recapitulate how the 6,L,E, applies to a quite different orientational problem namely, anisotropic rotational diffusion (10). 

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