On combining molecular dynamics and stochastic dynamics simulations to compute reaction rates in liquids

An approach that combines molecular dynamics and stochastic dynamics calculations for obtaining reaction rates in liquids is investigated by studying the cis-->trans isomerization of HONO in liquid krypton. The isomerization rates are computed for several liquid densities by employing full-dimensional molecular-dynamics simulations. The rates are also computed by employing the stochastic dynamics method for a wide range of collision frequencies. Comparisons of the two sets of the computed rates show that for a wide range of liquid densities there is a simple linear relation between the liquid density rho and the collision frequency alpha, that is, alpha=crho. This suggests that once the constant c is determined from a molecular-dynamics calculation at a single density, the reaction rates can be obtained from stochastic dynamics calculations for the entire range of liquid densities where alpha=crho holds. The applicability of the combined molecular dynamics and stochastic dynamics approach provides a practical means for obtaining rate constants at considerable savings of computer time compared to that required by using full-dimensional molecular-dynamics simulations alone.