Many rate processes like diffusion, dielectric relaxation, electron transfer reactions, nonradiative decay etc..., in dense media can be treated from a unified quantum statistical mechanical viewpoint [1]. In these unimolecular rate processes, one can regard that two potential surfaces, say a and b, are involved. It is assumed that when the two potential surfaces cross, the resonance interaction is small. A general theoretical approach has been developed that will treat these rate processes by properly choosing the perturbation associated with each rate process.