Thermodynamics of liquid metals: The hard-sphere versus one-compoent-plasma reference systems

Abstract

We have calculated the Helmholtz free energy of liquid Na and Al using a variational principle based on the Gibbs-Bogolyubov inequality in conjunction with hard-sphere and one-component-plasma reference systems. For liquid Na, the one-component plasma gives a lower variational bound to the free energy and fits the experimental structure factor better than does the hard-sphere fluid, in agreement with the findings of Ross et al. for liquid Li. Liquid Al, however, is better described by the hard-sphere reference system when exchange-correlation corrections to the pair potential are properly included. We ascribe the difference to the fact that the pair potentials of Al are relatively stiff, and those of the alkali metals relatively soft at small separations.