A theory of unimolecular chemical reactions in which the Rice–Ramsperger–Kassel theory was improved by the US chemist Rudolph Marcus (1923– ) in 1952. The improvement consisted of combining the original RRK theory with transition state theory. Further subsequent improvements by Marcus and his colleagues involved taking into account individual vibrational and rotational motion. The RRKM theory is very successful in explaining the results of experiments for unimolecular reactions for a variety of reactions.