Perturbation theory for fluids of non-spherical molecules in the presence of three-body forces

This work examines a correction to the Helmholtz free energy arising from anisotropy of pair and three-body non-additive interactions. The corrections are derived for axially symmetric molecules through the third order of a perturbation treatment. The effects of higher-order terms are then approximated by means of a simple Padé extrapolation procedure. A Lennard-Jones (12-6) potential is assumed to describe the spherically symmetric component of the two-body potential for overlap and dispersion interactions. The numerical results are given for the force parameters which are used to describe the gaseous properties of CO2, HCl and NH3 molecules.