Rice–Ramsperger–Kassel–Marcus (RRKM) theory

An improved form of Rice–Ramsperger–Kassel (RRK) theory in which account is taken of the way in which the various normal-mode vibrations and rotations contribute to reaction, and allowance is made for the zero-point energies. In this theory the energy ɛ * in an energized molecule is classified as either active, ɛ *active, or inactive, ɛ *inactive. The rate depends upon P ɛ *active N ɛ *, where N ɛ * is the density of states having energy between ɛ * and ɛ * + d ɛ *, and P ɛ *active is the sum of the active quantum states of the activated complex. This extension of RRK theory brings it in line with transition-state theory.
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 186