New Model for Classical Fluids. I

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Abstract
A simple new model for classical fluids is presented. Separate guesses are made at the form of the excess entropy and the excess internal energy. The two terms are made thermodynamically consistent by a minimization of the excess Helmholtz free energy. The two boundary conditions of yielding the exact equation of state of hard spheres and yielding an exact second virial coefficient for an arbitrary potential then completely determine the model, which does not contain any adjustable parameters. The model yields a critical point and van der Waals loops at subcritical temperatures for any potential containing an attractive region. Agreement with experimental data for argon and hydrogen is surprisingly good. Some predictions of this model for a Lennard‐Jones 6–12 potential are compared with real gases.