Thermal rate constant for H+CH3 → CH4 recombination. Comparison of quasiclassical trajectory and variational transition state theory

The variational transition state theory rate constant is calculated for the H+CH3 → CH4 bimolecular reaction and compared with the quasiclassical trajectory value reported previously. The calculations are performed on two different potential energy surfaces, MAPS/CH4-I and -II. To calculate the variational transition state theory rate constant, the reaction path and frequencies orthogonal to the reaction path are required. The general classical trajectory computer program mercury was modified to perform these calculations. Good agreement is found between the variational transition state theory and quasiclassical trajectory rate constants for both the MAPS/CH4-I and -II surfaces. The rate constants for the two surfaces differ by an order of magnitude. These calculated values are compared with a recent experimental measurement.