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Following our previous study of prototypical insertion reactions of energetically asymmetric type with the RPMD (Ring-Polymer Molecular Dynamics) method [Y. Li, Y. Suleimanov, and H. Guo, J. Phys. Chem. Lett. 5, 700 (2014)], we extend it to two other prototypical insertion reactions with much less exothermicity (near thermoneutral), namely, X + H[subscript 2] → HX + H where X = C([superscript 1] D), S([superscript 1] D), in order to assess the accuracy of this method for calculating thermal rate coefficients for this class of reactions. For both chemical reactions, RPMD displays remarkable accuracy and agreement with the previous quantum dynamic results that make it encouraging for the future application of the RPMD to other barrier-less, complex-forming reactions involving polyatomic reactants with any exothermicity., United States. Dept. of Energy. Office of Basic Energy Sciences (Massachusetts Institute of Technology. Energy Frontier Research Center for Excitonics. Combustion Energy Frontier Research Center. Award DE-SC0001198), MIT Energy Initiative

The kinetic isotope effect (KIE) of the seven-atom reactions OH + CH[subscript 4] → CH[subscript 3] + H[subscript 2]O and OH + CD[subscript 4] → CD[subscript 3] + HDO over the temperature range 200–1000 K is investigated using ring polymer molecular dynamics (RPMD) on a full-dimensional potential energy surface. A comparison of RPMD with previous theoretical results obtained using transition state theory shows that RPMD is a more reliable theoretical approach for systems with more than 6 atoms, which provides a predictable level of accuracy. We show that the success of RPMD is a direct result of its independence of the choice of transition state dividing surface, a feature that is not shared by any of the transition state theory-based methods. Our results demonstrate that RPMD is a prospective method for studies of KIEs for polyatomic reactions for which rigorous quantum mechanical calculations are currently impossible., United States. Dept. of Energy. Office of Basic Energy Sciences (Combustion Energy Research Fellowship Award DE-SC0001198), United States. Dept. of Energy (Grant DE-SC0001198)

The thermal rate coefficients and kinetic isotope effects have been calculated using ring polymer molecular dynamics (RPMD) for the prototypical reactions between methane and several hydrogen isotopes (H, D, and Mu). The excellent agreement with the theoretical rate coefficients of the H + CH[subscript 4] reaction obtained previously from a multi-configuration time-dependent Hartree calculation on the same potential energy surface provides strong evidence for the accuracy of the RPMD approach. These quantum mechanical rate coefficients are also in good agreement with the results obtained previously using the transition-state theory with semi-classical tunneling corrections for the H/D + CH[subscript 4] reactions. However, it is shown that the RPMD rate coefficients for the ultralight Mu reaction with CH[subscript 4] are significantly smaller than the experimental data, presumably suggesting inaccuracies in the potential energy surface and/or experimental errors. Significant discrepancies between the RPMD and transition-state theory results have also been found for this challenging system., United States. Dept. of Energy (Grant DE-FG02-98ER14914), United States. Dept. of Energy. Office of Basic Energy Sciences (Combustion Energy Research Fellowship Award DE-SC0001198)