The title theory provides means for calculating microcanonical and thermal rate constants of unimolecular dissociation and reverse radical association reactions of the type X + Y ↔ XY. It is intended for complex reactants for which only molecular parameters of reactant and products are available. The method used consists of first obtaining a partition function for the transition state by interpolation between total reactant and product partition functions. Inversion of this partition function yields a state count for the transition state as a function of distance along the reaction coordinate, which is then used in a variational routine. This theory has been used successfully to model and interpret experimental data on a variety of radical recombination reaction, an overview of which is presented and discussed.