METHOD FOR PREDICTING IVR-LIMITED UNIMOLECULAR REACTION-RATE COEFFICIENTS

We present a method based on diffusion theory (i.e., a classical maste r equation) for calculating unimolecular reaction rates at high energi es where reaction is limited by the IVR (intramolecular vibrational en ergy redistribution) rate. The method, which we refer to as intramolec ular dynamics diffusion theory (IDDT), uses short-time (a few fs) clas sical trajectory results to determine the characteristic times for the evolution of an initial microcanonical distribution, or, more specifi cally, the rate of IVR between the reaction coordinate and the ''bath' ' modes of the molecule. The IDDT method accurately predicts the rate of Si-Si bond fission in Si2H6 in the nonstatistical, IVR-controlled r egime, as demonstrated by comparisons with the results of a standard c lassical trajectory simulation. The method requires much less computer time than do the standard classical trajectory calculations. The meth od can be used to obtain results from the dynamical regime down to the statistical regime (near threshold), where rates can be calculated by Monte Carlo variational transition-state theory (MCVTST). Thus, the c ombined procedures can be used to calculate unimolecular reaction rate s in large molecules for realistic potential energy surfaces over the entire energy range. The main approximation is the assumption of class ical mechanics. (C) 1997 American Institute of Physics.