High-temperature free-energy expansion for metal fluids

The cluster expansion of the Helmholtz free energy is based on two-body interactions through a central potential phi(r). For a fluid at fixed density, it is shown that the cluster expansion provides a convergent high-temperature expansion of the free energy. Because phi(r) for a liquid metal depends explicitly on the density, the corresponding cluster expansion of the pressure differs from the ordinary virial series. For a physically realistic interaction, phi(r) is integrable at small r. From this it follows that the two-body-interaction contributions to thermodynamic functions for realistic fluids are bounded at high temperatures. In contrast, for potentials such as Lennard-Jones which are not integrable at small r, these thermodynamic contributions increase without limit as the temperature increases