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. (C) 2004 American Institute of Physics.