AB-INITIO INTERMOLECULAR POTENTIALS OF METHANE, NITROGEN AND METHANE PLUS NITROGEN AND THEIR USE IN MONTE-CARLO SIMULATIONS OF FLUIDS AND FLUID MIXTURES

Intermolecular pair potentials of methane and of methane + nitrogen have been calculated by quantum chemical ab initio methods. The repulsive and electrostatic parts were determined pointwise for various distances and orientations of the dimers (supermolecule approach) by self-consistent field (SCF) calculations including the counterpoise correction. Gaussian basis functions of triple-zeta quality plus one set of polarization functions on all atoms were used. The dispersion energy, which cannot be calculated at the SCF level, has been added by a semi-empirical estimate. These potentials and the pair potential for nitrogen (of similar quality, taken from the literature) were fitted to analytical functions and used for NVT Monte Carlo simulations of thermodynamic properties of the fluids and their mixture over a wide temperature/density area. Comparison with measurements and with Monte Carlo results from the literature (pressure, internal energy, radial distribution function) obtained with other pair potentials indicates the quality of the present calculations.