We present a method based on diffusion theory (i.e., a classical master equation) for calculating unimolecular reaction rates at high energies where reaction is limited by the IVR (intramolecular vibrational energy redistribution) rate. The method, which we refer to as intramolecular dynamics diffusion theory (IDDT), uses short-time (a few fs) classical trajectory results to determine the characteristic times for the evolution of an initial microcanonical distribution, or, more specifically, 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 regime, as demonstrated by comparisons with the results of a standard classical trajectory simulation. The method requires much less computer time than do the standard classical trajectory calculations. The method 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 combined procedures can be used to calculate unimolecular reaction rates in large molecules for realistic potential energy surfaces over the entire energy range. The main approximation is the assumption of classical mechanics. (C) 1997 American Institute of Physics.