Diagram technique in an improved nonsymmetrized self-consistent field method

The free energy of a strongly anharmonic crystal is expanded in powers of a small parameter which is the ratio between the mean amplitude of thermal atom vibrations and the interatomic spacing, where the main contribution of the principal anharmonic terms is included in the zeroth approximation (the nonsymmetrized self-consistent field approximation). A diagram representation is used to calculate the corrections. As an illustration of the method, the corrections to the free energy of a strongly anharmonic monatomic linear chain are obtained. © 1984 Plenum Publishing Corporation.