The influence of the form of the interaction potential on the thermodynamic properties of fluids is investigated. Differences in the potential profile of nonconformal interactions are taken into account by the steepness of the potential functions and used to define a relative softness between interactions. In the dilute gaseous phase, the system of interest is characterized by a relative softness relating its virial coefficient B(T) with respect to that of a reference system B 0 (T). For constant softness S, B(T) is obtained directly from B 0 (T) by linear relations involving only S. The conditions on which S is exactly or approximately constant are analyzed and shown to hold in a wide range of cases. Furthermore, it is shown how to invert B(T) to construct potentials whose form and parameters reproduce accurately the thermodynamic properties of the gas.

The approximate nonconformal (ANC) theory recently proposed has been very successful in determining interaction potentials for the noble gases and their mixtures. The ANC theory is used here to obtain e †ective angle averaged potentials of all homodiatomic gases for which experimental second virial coefficient data are available :and The cross virial coefficients in the mixtures of homodiatomics among. themselves and with noble gases are predicted with excellent agreement with experiment for the heavier classical gases. The atomÈatom interactions, which should be an improvement over previous results, are also determined and shown to behave regularly with atomic number. The critical temperatures and volumes of these gases vary smoothly when scaled with the parameters of the ANC potential.

A theory recently proposed characterizes effective two-body interactions in gases by molecular sizes and energies plus effective measures of the nonconformality between the exact potential and a spherical reference. This theory provides a procedure to construct effective potentials which reproduce the second virial coefficients B(T) of the substance of interest and allows us to express B(T) in simple and compact form. In this paper we test the applicability of the theory to a variety of nonspherical modelssspherocylinders, ellipsoids, Lennard-Jones polyatomics, square-well chains, and Stockmayer moleculessand show that their virial coefficients are accounted for accurately by the theory. The theory is used to study the effects of molecular geometry, particularly the elongation of linear molecules, on the angle averaged molecular parameters and on the effective potential. The effective potentials of the models considered are obtained, and the effects of size and geometrical shape are discussed.

The recently proposed approximate nonconformal (ANC) theory has been very successful in determining effective interaction potentials for a large number of pure substances in the gaseous state: noble gases, homodiatomics, alkanes, perfluoroalkanes and some small polyatomic molecules, as well as many of their binary mixtures. ANC potentials are spherically symmetric Kihara-like functions involving an energy ", a diameter and a form parameter s. In this work, we propose an effective-potential theory valid at finite densities. The basic assumption is that ANC potential functions represent effectively the thermodynamics of a substance over the isotropic fluid region given the appropriate state dependencies of ", and s. The theory proposes a relation between the critical temperature of a substance and " that is used here to predict effective potentials for the one-carbon Freons: CCl a collection of small molecules that are all of similar geometry but varying degrees of polarity. As a test of the reliability of the theory the calculated second virial coefficients B(T ) are compared with experiment. For most of these substances, B(T ) is reproduced within experimental error. The relative contributions of the polar and nonpolar parts of the potential to the thermodynamics are discussed. The approach produces effective interactions and generalized Stockmayer potentials for this set of molecules to be used in predicting other thermodynamic properties.

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