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 ha ve been calculated by quantum chemical ab initio methods. The repulsiv e and electrostatic parts were determined pointwise for various distan ces and orientations of the dimers (supermolecule approach) by self-co nsistent field (SCF) calculations including the counterpoise correctio n. Gaussian basis functions of triple-zeta quality plus one set of pol arization functions on all atoms were used. The dispersion energy, whi ch cannot be calculated at the SCF level, has been added by a semi-emp irical estimate. These potentials and the pair potential for nitrogen (of similar quality, taken from the literature) were fitted to analyti cal functions and used for NVT Monte Carlo simulations of thermodynami c properties of the fluids and their mixture over a wide temperature/d ensity area. Comparison with measurements and with Monte Carlo results from the literature (pressure, internal energy, radial distribution f unction) obtained with other pair potentials indicates the quality of the present calculations.