FLUX FACTORS IN VARIATIONAL TRANSITION-STATE THEORY

The role of the flux, or velocity, factor in variational transition-state theory (VTST) with a generalized reaction coordinate is examined within the context of barrierless recombination reactions. It is found that well-behaved approximations to the rate coefficient can be obtained by allowing more flexibility in the choice of the flux factor. Transition-state theory traditionally prescribes a flux factor which is the velocity perpendicular to the dividing surface. We examine two models for the application of VTST to barrierless recombination reactions with a bond-length reaction coordinate: one in which the flux factor is the rate of change of the bond length (i.e., perpendicular to the dividing surface) and the other in which the flux factor is the rate of change of the separation between the fragment centers of mass. The bond-length flux factor produces results in agreement with those of Klippenstein's model [J. Chem. Phys. 1991, 94, 6469], reducing the computation time by approximately 2 orders of magnitude. The center-of-mass flux factor, which can be implemented variationally, also produces excellent results and has the additional advantage of correlating correctly to the number of asymptotically open channels predicted by phase space theory (PST) at large separation of the fragments, where the variational transition state typically lies for energies close to threshold.