Rate constants from Monte Carlo quasi-classical trajectory calculations. A procedure for importance sampling

The use of importance sampling as a variance reduction technique is applied to the direct Monte Carlo quasi-classical trajectory (MCQT) calculation of bimolecular gas phase rate constants. A general importance sampling function for the collision energy having a single (integer) parameter which characterizes the distribution of collision energies to be sampled is proposed and used in calculations of rate constants for three chemical reactions [F + H₂, Cl + H₂, and O(³P) + H₂] at several temperatures. Results are compared with regard to accuracy and efficiency with rate constants obtained directly or indirectly using other MCQT approaches and the same potential energy surfaces. The importance sampling calculations are found to be more efficient over the entire range of reactivity studied, and provide statistically valid estimates of the errors in the computed rate constants.