Gaussian Approximation for Ising Model. Variational Approach

Abstract

A variant of a Gaussian approximation in Ising model of phase transition is suggested. The method of functional integration with the application of a variational method is put in the basis of the approach. The Hamiltonian (the functional) is presented which in the method of functional integration produces the known results of a two-tails approximation obtained by a diagram method. The application of a variational method eliminates a principal demerit of the two-tails approximation -incorrect description of the phase transition order. The results of numerical calculations for two and threedimensional lattices are presented.