Ordering energy of B2 alloys calculated in the frozen potential and Harris approximations

In this paper it is established that the coherent potential approximation can successfully describe the energy of random alloys. It also served as the basis of generalized perturbation method and concentration wave calculations of the energy of short range ordered alloys. The multisublattice coherent potential, (MCPA) is the natural extension of the CPA with which to address long range order (LRO). Using the recently developed multi-sublattice coherent potential approximation Korringa Kohn Rostoker, (MCPA-KKR) code the eigenvalue sum can be calculated as a function of LRO. This allows the evaluation of the ordering energy by either of two approximations. The frozen potential approximation (FPA) assumes that the muffin-tin single site potentials do not change as the long range order is varied; the Harris Approximation, (HA) as applied in this work, assumes that the single site charge densities do not change as the long range order is changed. These two methods of calculating the ordering energy will be compared with each other and to experiment for several systems including CuZn, NiAl, and NiAl with zinc additions