Second virial coefficients and the simulation of equilibrium properties of liquid nitrogen using model potentials

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Abstract
Computer simulations of liquid equilibrium properties and calculations of the gas second virial coefficient have been made using two of the available representations of the nitrogen intermolecular potential of Berns and van der Avoird. One representation (BV) employs a spherical harmonic expansion and the other (BVAA) employs an atom-atom representation of the same potential energy input data. A variant potential (BV(+)) in which the isotropic dispersion coefficients C 8 and C 10 are increased by 10 per cent and 15 per cent, relative to those in the BV potential, has also been studied. The virial coefficients for BV(+), but not for BV and BVAA, do not differ from experiment by more than the estimated experimental errors and there are significant differences between the BV(+), BV, and BVAA values. The liquid simulation results for the thermodynamic properties and radial distribution functions are compared with each other and with literature values obtained from simulations using effective potentials which employ a two centre Lennard-Jones potential plus quadrupole interaction. At given temperature and density the internal energy and pressure differ significantly for BV and BVAA. Although neither is as successful in comparison with experiment as the best effective potentials, the differences between them are not greater than those encountered between extreme results for the different effective potentials. We emphasize that, in contrast to the effective potentials, the BV and BVAA potentials contain no parameters which have been fitted using bulk properties.