On the difference between a point multipole and an equivalent linear arrangement of point charges in force field models for vapour-liquid equilibria; partial charge based models for 59 real fluids

The replacement of a point dipole and a point quadrupole by a corresponding linear arrangement of two point charges (+q, -q) and accordingly three point charges (+q, -2q, +q) is studied with respect to vapour-liquid equilibria. The dependence of saturated liquid density, vapour pressure and heat of vaporization on the choice of the distance d between the charges in the point charge arrangement is analysed. For the studied dipolar two-centre Lennard-Jones (2CLJD) and quadrupolar two-centre Lennard-Jones (2CLJQ) models, d/sigma between 1/15 and 1/20 is a reasonable compromise between numerical and physical accuracy, where sigma is the Lennard-Jones size parameter. The results are used to derive validated partial charge based models of 59 real fluids from previously published point dipole and point quadrupole based models.