A Model for Sub-critical Liquid-Vapour Coexistence in Sodium

The Hamiltonian of the ions in liquid sodium is obtained from pseudopotential perturbation theory, and the free energy is evaluated using the Gibbs-Bogoliubov variational technique together with a hard sphere reference system. The free energy of the vapour is obtained by supposing it to be a monomer-dimer mixture. The two descriptions are then combined to provide a theory of coexistence. This yields satisfactory results for the various thermodynamic properties up to a few hundred degrees short of the critical temperature.