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 ɛc is localized in one particular degree of freedom.
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 185 (https://doi.org/10.1351/pac199668010149)