Order of phase transition on a simple cubic lattice determined by cluster variation method

Detailed analyses of the free energy behavior near the order-disorder transition temperature on a simple cubic lattice are attempted by cluster variation method (CVM). The entropy is formulated within a cubic approximation of the CVM and two kinds of nearest neighbor pair interaction energies are assumed in the internal energy; one is independent of an atomic distance, r, and the other depends on r through Lennard-Jones type pair potential. In both cases, it is found that the order of the transition is of second order.