A VARIATIONAL APPROXIMATION FOR CUBIC LATTICE MODELS IN STATISTICAL-MECHANICS
RJ BAXTER, PJ FORRESTER
JOURNAL OF PHYSICS A-MATHEMATICAL AND GENERAL | IOP PUBLISHING LTD | Published : 1984
A variational approximation applicable to three-dimensional isotropic cubic lattice models is formulated. When applied to the simple cubic, face centred cubic and body centred cubic Ising models, the approximation correctly gives the first 14, 19, and 23 terms respectively of the known low-temperature free energy expansion.