Quasi-adiabatic propagator path integral methods. Exact quantum rate constants for condensed phase reactions

We describe an exact quantum-mechanical methodology for calculating Boltzmann-averaged rate constants for a system coupled to a harmonic bath of arbitrary dimensionality. Reactive flux correlation functions are evaluated using the quasi-adiabatic propagator path integral (QUAPI) method. Extremely rapid convergence is achieved: i.e. exact results are obtained with three to five time slices even at temperatures as low as 100 K, and the numerical effort required is essentially independent of the number of bath degrees of freedom. Thus, the proposed method provides an accurate and efficient scheme for studying quantum effects in the low-temperature kinetics of condensed phase reactions.