Nonconformal potentials and second virial coefficients in molecular fluids. 1. Theory

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 th e potential functions and used to define a relative softness between intera ctions. In the dilute gaseous phase, the system of interest is characterize d 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 obta ined directly from B-0(T) by linear relations involving only S. The conditi ons 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 t he thermodynamic properties of the gas.