Anharmonic coupling theory parameters

Since the PE surfaces are anharmonic in this limit, the Franck-Condon contributions can be difficult to treat. The approaches to this limit can be separated into two classes (a) perturbation theory corrections of the weak-coupling limit and (b) quantum-mechanical calculations of the reaction coordinate. The latter tend to be done on a reaction-by-reaction basis this makes it difficult to generalize. The perturbation theory approaches have the advantage that they make use of the parameters used in the weak-coupling limit, and this can provide usefiil insights into general trends and patterns. 

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