Comparison of molecular dynamics and variational transition-state-theory calculations of the rate constant for H-atom association with the diamond {111} surface

The quasiclassical trajectory method was used to study the dynamics and kinetics of H-atom association with a C-atom radical site on the diamond {111} surface. The calculations employed an analytic potential-energy surface derived previously [P. de Sainte Claire, P. Barbarat, and W. L. Hase, J. Chem. Phys. 101, 2476 (1994)] from high-level ab initio calculations. The trajectory rate constant calculated here of 1.7±0.3×${10}^{13}$ ${\mathrm{cm}}^3$ ${\mathrm{mol}}^{\mathrm{-}1}$ ${\mathrm{s}}^{\mathrm{-}1}$ for temperatures of 1000 and 2000 K is in excellent agreement with the one calculated previously on the same analytic potential-energy surface using canonical variational transition-state theory. For H atoms impinging perpendicularly and directly onto the C-atom radical site, the association probability is sensitive to the frequencies of the diamond lattice and the treatment of the lattice’s zero-point energy. However, trajectories with this orientation make a negligible contribution to the thermal rate constant, which is found to be nearly insensitive to the lattice frequencies and zero-point energy. Trajectories, for which H atoms move toward the surface with a small angle relative to the surface plane and pass above the C-atom radical site before associating, make an important contribution to the association rate constant.