Modified cluster-field method for statistics of ordering systems

A relatively simple cluster approximation for the investigation of the phase transition thermodynamics in order-disorder systems is proposed. The approximation is demonstrated on the square and simple-cubic Ising lattices with nearest-neighbour ferromagnetic coupling. The accuracy of the approximation is shown to be no worse than that of the well known cluster-variation method (CVM). At the same time the modified cluster-field method has fewer variational parameters and is easier to use than the CVM, allowing use of larger clusters for better accuracy.