ANALYTICAL CHARACTERS OF 2-DIMENSIONAL ISING-MODEL WITH VARIATIONAL-CUMULANT EXPANSION TO THE 7TH ORDER

In order to investigate the variational-cumulant expansion (VCE) metho d, the 2-dimensional Ising model is expanded to the seventh order by a dopting the method of enumerating connected diagrams, in whose process , how to determine the variational parameter is analyzed. The internal energy (U), specific heat (C) and spontaneous magnetization (M) are c alculated from the first order to the seventh order. The results show that the thermodynamic and magnetic quantities converge to the exact v alues very quickly and the critical point approaches the exact one ord er by order. The curve-pairing phenomenon is watched between neighbori ng order curves at high temperature region.