Quasiclassical trajectories and the H + CH4 reaction threshold

CHED 1435

James C. Shearouse, Zhen Xie, Kurt M. Christoffel, and Joel M. Bowman. Emory University, Cherry L. Emerson Center for Scientific Computation, Department of Chemistry, 1515 Dickey Drive, Atlanta, GA 30322
Quasiclassical trajectories are a commonly used technique for studying the gas-phase reactions of small molecules. As this technique requires integrating Newton's equations of motion rather than solving the time-dependent Schrödinger equation, the possibility of several nonphysical results exists. Perhaps the most important of these phenomena is zero point energy (ZPE) leakage. In the H + CH4 system the ZPE of the products is significantly higher than that of the CH5 transition state, so a certain amount of ZPE leakage is in fact physically desirable during transition state formation. However, our calculations seem to suggest that the energetic threshold of the reaction is about half of the adiabatic threshold as determined from the ZBB-2 potential energy surface. Removal of ZPE from various normal modes (to prevent leakage) does not fix the problem in this case.