Inadequacy of the Lorentz-Berthelot combining rules for accurate predictions of equilibrium properties by molecular simulation

Though molecular beam experiments have revealed deficiencies in the Lorentz -Berthelot combining rules, these rules are still used widely to parametriz e effective pair potential models or to calculate the thermodynamic propert ies of mixtures. Gibbs ensemble Monte Carlo and isothermal isobaric Monte C arlo simulations were used to compute the liquid-vapour phase equilibria an d the liquid properties of binary mixtures of rare gases modelled by effect ive pair potentials. Three sets of simple combining rules were tested in th is work: the commonly used Lorentz-Berthelot rules, the Kong rules (Kong, J ., 1973, J. chem. Phys., 59, 2464) and the Waldman-Hagler rules (Waldman, M ., and Hagler, A. T., 1993, J. comput. Chem., 14, 1077). These three sets o f rules do not require any additional parameter. It is shown that: (1) the choice of a set of combining rules has a significant effect on the thermody namic properties, (2) using the Lorentz-Berthelot rules yields significant deviations from experiment and (3) the Kong rules provide a much better des cription of the mixture properties both for coexistence curves and liquid p roperties. We therefore recommend the use of the Kong rules instead of the Lorentz-Berthelot when parametrizing potential models.