MONTE-CARLO CALCULATIONS FOR METHANE AND ARGON OVER A WIDE-RANGE OF DENSITY AND TEMPERATURE, INCLUDING THE 2-PHASE VAPOR-LIQUID REGION

The Gibbs-ensemble simulation technique provides a powerful method to calculate vapor-liquid phase behavior [1). To evaluate the configurational energy of a system of molecules, commonly used experessions describe the interaction between two molecules. Contributions from higher-body forces are usually implicitly taken into account by adjusting two-body potential parameters to give agreement with experimental data. Explicit expressions for higher-body potentials are not commonly used in simulations [8]. The work by Smit et al. [9] gives the appropriate expressions to evaluate the pressure as well as the chemical potential from a density-dependent two-body potential in an NVT ensemble. In the present work, contributions to the potential from two-body interactions are separated from those due to higher-body interactions; to take higher-body forces into account, a mean-field term, proportional to (density)0.9, is added to the two-body potential. NPT-simulations over a wide range of temperature and density, as well as Gibbs-ensemble simulations, are used to evaluate phase behavior of argon and of methane. The results indicate that a simple mean-field correction to the ''true'' two-body Kihara potential provides good agreement between experiment and simulation.