Atomistic approach to ordering

The capability of the Ising model in coordinating various ordered structures of alloys and compounds is discussed on the basis of the analysis of the ground states. Vacancy ordering in the naCl type compounds and fcc ordered alloys of transition metals are illustrative examples of the discussion. In the latter example the appearance of incommensurate or long period modulated structures is discussed in connection with commensurate structures. The phenomenological analysis is followed by a discussion of the calculation of the interaction energy of atoms based on the electronic structure of alloys.