Comparison of ab Initio and Empirical Potentials for H-Atom Association with Diamond Surfaces

Pascal de Sainte Claire, Kihyung Song, William L. Hase, Donald W. Brenner
1996 The Journal of Physical Chemistry  
Canonical variational transition-state theory (CVTST) is used to compare H + CH 3 and H + diamond {111} association rate constants calculated from the Brenner empirical potential function and molecular anharmonic potentials written with switching (MAPS) functions. Previous work [J. ] has shown that the MAPS functions, derived from ab initio calculations, give rate constants in agreement with experiment. For the 300-2000 K temperature range, the Brenner potential function gives CVTST H + CH 3
more » ... H + diamond {111} association rate constants which are 159-30 and 49-7 times smaller, respectively, than the values from the MAPS functions. An analysis of the Brenner potential function shows that it inaccurately represents the intermediate and long-range H---C association potential, which controls the structure of the variational transition state and the CVTST rate constant. The MAPS functions give H + CH 3 and H + diamond {111} variational transition states with similar properties. Angular momentum and external rotation have no effect on the H + diamond {111} association rate constant, which makes it approximately an order-of-magnitude smaller than that for H + CH 3 association. † Current address: Computational Procedure Potential Energy Surfaces. The expressions and parameters for the Brenner general hydrocarbon potential have been given previously, 29,44 from which it is straightforward to construct the Brenner potentials for H + CH 3 and H + diamond {111}. The MAPS/CH 4 potential was originally developed by Duchovic and co-workers 39 and then modified by Hase et al. 40 Its next and last modification was by Hu and Hase, 41 which is the form used here. Details of the MAPS/HDIAM potential have been described in recent work. 26 Reaction Path and Canonical Variational Transition State Theory (CVTST). The potential energy functions studied here are incorporated in the general chemical dynamics computer program VENUS. 45 Determining the reaction path and canonical variational transition state, for an analytic potential, is a standard option in VENUS. The reaction path is determined by following the path of steepest descent in mass-weighted Cartesian coordinates. 46 This gives the molecular geometry and potential energy as the system moves along the reaction path. Harmonic vibrational frequencies, for the 3N -7 modes orthogonal to the reaction coordinate, are also determined as the system moves along the reaction path, by diagonalizing a projected 3N × 3N mass-weighted Cartesian force constant matrix. 47 Projected out of the standard force constant matrix 48 are the reaction coordinate motion and six infinitesimal displacements for overall translation and rotation. The canonical variational transition state is placed at the free energy maximum along the reaction path. 6,7,49-51 The free energy, as a function of the reaction path, is written as
doi:10.1021/jp951693m fatcat:tcit4c6hbbculnxwbphiws75nm