A CLOSED-FORM APPROXIMATION FOR THE 3-DIMENSIONAL ISING MODEL

Abstract

An analytical expression for the partition function of the 3-dimensional Ising Model is obtained (see formula (32)). It is approximate, but has several rational features: (1) It is valid on the whole temperature range from zero to infinity and has only one singular point at uc = 0.2612 in comparision to 0.22 from the series expan-sion; (2) It leads to a nonlogarithmic singularity for the specific heat; (3) It can be reduced into the exact Onsager 2-dimensional solution for planar rectangle lattice in the anisotropic case (see formula (34)) ; (4) It has a high temperature expansion, the first two coefficients are correct and the higher ones diminish systematically but remain positive.