Perturbation theory of non-polar convex molecule fluids

The second-order perturbation expansion is proposed for describing the equilibrium behaviour of non-polar fluids whose intermolecular interactions are expressed by the Kihara non-central pair potential. The definition of the predominant action range of the attractive and repulsive forces as well as the expression for the first-order perturbation term correspond to the WCA formulation for particles with central interactions; to determine the second-order perturbation term the extended macroscopic compressibility approximation is used. The system of soft convex bodies parallel to molecule cores is taken as the reference system; its equilibrium behaviour is described by thermodynamic functions of representative hard convex bodies with temperature and density dependent thickness. For calculating these functions, a very accurate equation of state was used; the average correlation function was approximated on the basis of the correlation function of corresponding hard spheres. The perturbation method was used for determining thermodynamic functions of nitrogen and for studying its phase behaviour; the calculated coexistence curve is compared with experiment.