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

The application of perturbation theory to a fluid of non-spherical molecules of arbitrary symmetry is considered. The influence of a large number of angle-dependent pair interactions and three-body non-additive interactions on the Helmholtz free energy and on the pair and triplet distribution functions, in some cases through the third order of the perturbation theory, is evaluated. Explicit expressions are given for the Helmholtz free energy for molecules possessing axial and tetrahedral symmetries in terms of molecular parameters and the reference systems pair and triplet distribution functions. The contributions of the various terms discussed in this paper are expected to be substantial at low temperatures for a real fluid.