  Thermal rate constants and several kinetic isotope effects were evaluated for
the OH + CH4 hydrogen abstraction reaction using two kinetics approaches, ring
polymer molecular dynamics (RPMD), and variational transition state theory with
multidimensional tunnelling(VTST/MT), based on a refined full-dimensional
analytical potential energy surface, PES-2014, in the temperature range
200-2000 K. For the OH + CH4 reaction, at low temperatures, T = 200 K, where
the quantum tunnelling effect is more important, RPMD overestimates the
experimental rate constants due to problems associated with PES-2014 in the
deep tunnelling regime and to the known overestimation of this method in
asymmetric reactions, while VTST/MT presents a better agreement, differences
about 10%, due to compensation of several factors, inaccuracy of PES-2014 and
ignoring anharmonicity. In the opposite extreme, T = 1000 K, recrossing effects
play the main role, and the difference between both methods is now smaller, by
a factor of 1.5. Given that RPMD results are exact in the high-temperature
limit, the discrepancy is due to the approaches used in the VTST/MT method,
such as ignoring the anharmonicity of the lowest vibrational frequencies along
the reaction path which leads to an incorrect location of the dividing surface
between reactants and products. The analysis of several kinetic isotope
effects, OH + CD4, OD + CH4, and OH + 12CH4/13CH4, sheds light on these
problems and confirms the previous conclusions. In general, the agreement with
the available experimental data is reasonable, although discrepancies persist,
and they have been analysed as a function of the many factors affecting the
theoretical calculations: limitations of the kinetics methods and of the
potential energy surface, and uncertainties in the experimental measurements.
