The quasiharmonic and lowest order anharmonic contributions to the Helmholtz free energy of sodium chloride have been calculated. A breathing shell model was used in the quasiharmonic calculation, and Coulomb and polarization contributions to the anharmonic matrix elements were included more completely than has been done before. The thermodynamic properties derived from the free energy are in good agreement with experimental results. The inclusion of Coulomb anharmonicity was particularly important, reducing the magnitude of the second order anharmonic contribution by 40%. The quasiharmonic calculation gives good agreement with experiment up to one quarter of the melting temperature, and the lowest order anharmonic theory appears to be adequate up to at least two thirds of the melting temperature.
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E. R. Cowley (1971) studied this question.