Abstract
In the unsymmetrized self‐consistent field approximation the second‐order correction to the Helmholtz free energy is derived by statistical perturbation theory. This correction takes into account the effect of the anharn micity of odd orders on the Helmholtz free energy and on the thermodynamic properties of a crystal. The case of the linear chain is studied in more detail. Special attention is paid to the case of strong anharmonicity.