Rice–Ramsperger–Kassel (RRK) theory

A theory of unimolecular gas reactions in which the rate with which the energized reactant molecule breaks down is treated as a function of the energy $ɛ$ that it contains. The theory assumes that the rate is proportional to the number of ways of distributing $ɛ$ among the internal degrees of freedom of the reactant molecule, in such a manner that the critical energy $ɛ c$ is localized in one particular degree of freedom.

Source:

PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 185