A statistical rate constant calculation method based on Monte Carlo transition state theory and application to unimolecular dissociation of HFCO

A statistical calculation method is proposed for a classical reaction rate constant expressed as the equilibrium flux of systems through a dividing surface that corresponds to the hypersurface at the transition state perpendicular to the reactive normal coordinate. In this method, an efficient microcanonical Monte Carlo sampling method is employed to approximately compute the flux integral for the classical rate constant expression. To determine whether or not each configuration generated by a Markov walk in the Monte Carlo sampling lies on the dividing surface, we present a method to evaluate the value of the reactive normal coordinate at the configuration by means of a transformation of internal coordinates into normal coordinates. We applied the present statistical calculation method to the unimolecular dissociation of HFCO. It was found that the calculated rate constant is much more correct than the rate constant evaluated as the equilibrium flux through an intuitive dividing surface specified by an HCF angle. In addition, a separable and harmonic approximation for vibrational-rotational modes on a rate constant was found to overestimate the rate constant for the dissociation. (C) 1999 American Institute of Physics. [S0021-9606(99)00506-1].