Cell-cluster and self-consistent calculations for a model sodium chloride crystal

Abstract

We have calculated the thermodynamic properties of a simple rigid-ion model of sodium chloride, using both cell and cell-cluster, and self-consistent- and improved self-consistent-phonon formalisms, to compare with recent Monte Carlo results. The cell-cluster results are good for the thermal expansivity and the bulk modulus, which involve the volume dependence of the free energy, but some discrepancies remain for the heat capacities. The improved self-consistent theory gives a better result for ${\mathit{C}}_{\mathit{v}}$, although the volume-dependent properties are less well described. The simple cell model is surprisingly successful in giving a description of the solid at high temperatures.