Thermodynamics and structure of liquid metals: the variational approach versus the optimised random phase approximation

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Abstract
The author compares the thermodynamic excess quantities for nine liquid metals which are calculated from the pseudopotential-induced interionic potential and the thermodynamic perturbation methods from the hard-sphere fluid. He considers two useful and widely used methods: the variational approach (VA) and the optimised random phase approximation (ORPA). The latter method allows the determination of the deviation of the liquid metal structure from the hard-sphere one, but not the VA. He shows that the corresponding change of the structure energy is not well described by the variational procedure. In the alkalis, the difference between the VA and the ORPA comes form the estimation of the short-ranged repulsive forces, as in the case of the Lennard-Jones fluid, but in the polyvalents metals the Friedel oscillatory forces also make a significant contribution. He points out the usefulness of the ORPA for the analysis of the thermodynamic stability of the liquid structure and he shows that the distortions of the liquid metal structure discussed recently by J. Hafner and G. Kahl (1984) are energetically favourable.