Reliable ab initio calculation of a chemical reaction rate and a kinetic isotope effect: H + H(2) and H + H(2).

We calculate equilibrium rate constants for ortho-para conversion in hydrogen and deuterium by an atomic mechanism. The calculations are based on an accurate ab initio potential surface, transition state theory, and an adiabatic transmission coefficient. The calculated rate constants are demonstrated to be reliable within 40-50%, and they agree with experiment within this margin.