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, for the system on the square and the sc lattice. These values are nearly equal to the mean values of those in the pair (Bethe) and the square (Kikuchi) approximation. The phase diagram in the square approximation of the present scheme is compared with other approximations and the exact result for a frustrated Ising model on the centred square lattice.
