Effective interatomic interactions in inhomogeneous semi-infinite systems

The parameters entering the effective Ising model of disordered alloys with nonuniform composition at the sample surface are derived within the framework of the generalized perturbation method. The tight-binding version of the linear-muffin-tin-orbital method and its generalization to inhomogeneous alloys is used to describe the electronic structure in the local-density approximation, while the semi-infinite nature of the problem is included via the surface Green's-function approach. The method is applied to evaluate the site-diagonal terms of the Ising model as well as the effective interatomic interactions between sites within and between top layers up to the fourth-nearest neighbors of the fcc (001) face of the transition-metal alloys Pd₅₀{Rh}₅₀$ and ${Ag}₅₀Pd₅₀.