Spiers Memorial Lecture - Quantum and semiclassical theory of chemical reaction rates

Transition state theory (TST) has provided the qualitative picture of chemical reaction rates for over sixty years. Recent theoretical developments, however, have made it possible to calculate rate constants fully quantum mechanically and efficiently, at least for small molecular systems; vestiges of TST can be seen both in the resulting flux correlation functions and in the algorithmic structure of the methodology itself. One approach for dealing with more complex molecular systems is the semiclassical (SC) initial value representation (IVR), which is essentially a way of generalizing classical molecular dynamics simulations to include quantum interference; electronic degrees of freedom in an electronically non-adiabatic process can also be included on a dynamically equivalent basis. Application of the SC-IVR to models of unimolecular isomerization and of electronically non-adiabatic transitions, both coupled to an infinite bath of harmonic oscillators, gives excellent agreement with (essentially exact) quantum path integral calculations for these systems over the entire range of coupling strength.