Morita, T. - In: Physica A: Statistical Mechanics and its Applications 155 (1989) 1, pp. 73-83
A variational calculation is given of an approximation scheme, which is far easier to apply than the cluster variation method, in the higher approximations. The Curie temperature of the Ising model in the square approximation in the present scheme is obtained as 2.6253 and 4.7611, respectively,...