A general formalism is presented for the computation of the longitudinal magnetization in a broad class of molecular crystals allowing for the effects of cross correlations between the motions of different spinndash;spin vectors. These effects will manifest themselves, in general, in a departure from exponentiality of the relaxing magnetization as a function of time. The molecular motion concerned is that of random, thermallyhyphen;activated. hindered reorientations about the available symmetry axes. Any combination of available reorientations is covered by the theory, provided only that the motion preserves the complete equivalence of all the rij. The present work is essentially concerned with outlining the principles of the calculation, but to demonstrate the formalism, one simple example, amenable to analytic solution, is examined in detail.
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