The thermodynamic functions of molecular liquids in the interaction site model

The excess Helmholtz Free Energy (H.F.E.) of the hard molecule liquid with respect to the perfect gas (point-like non-interacting molecule) is obtained; the Equation of State (E.S.) is then derived. The site-site and molecular correlation functions involved in both the excess H.F.E. and the E.S. are determined in the framework of a modified Reference Interaction Site Model. For a symmetric diatomic molecule the exact value g 11(r = σ+) = 1/4 of the site-site correlation function and the exact second virial coefficient is obtained.