VARIATIONAL APPROACH TO MELTING .2.

A variational approach to the equilibrium thermodynamic properties of an original system based upon an inequality for the Helmholtz free energy of that system is introduced. A system with molecules obeying the cell model of Lennard-Jones and Devonshire, and having a harmonic-oscillator-type potential function inside their cells, is used for a reference system to produce the inequality for the Helmholtz free energy of the original system. Optimization upon this inequality indicates that a variational calculation based on a reference system with highly ordered structure, as the cell model, predicts the properties of the solid phase better than the liquid phase. Also, it shows that by an ordered-structure reference model, it is possible to predict the liquid-solid phase transition. Equilibrium thermodynamic properties of the solid phase and liquid-solid phase equilibria are calculated and are compared with the machine-calculated and the experimental data.