Here, we develop an analytic theory to describe the collective PMET rate constant. The details of the derivations are provided in the Supporting Information. For a reaction occurring across a large number of nuclear coordinates R among a set of electronicphotonic states, a collective reaction coordinate R RC ab (X) from state |a⟩ to state |b⟩ can be defined as a function of the energy difference X. For a fixed X = ∆E ab (R), the collective reaction coordinate R RC ab (X) is the nuclear configuration that minimizes the donor energy E a (R). This collective reaction coordinate can be used to derive reaction parameters such as the driving force and reorganization energy.