Ab initio study for magnetism in Ni2MnAl full-Heusler alloy: A cluster expansion approach for total energy

We show the accuracy and convergence of our ab initio real-space cluster expansion (CE) approach for total energies of alloys, which is useful for the study of atomic structures and magnetism of A-rich AXY full-Heusler alloys such as Ni2MnAl. In the present CE, each and every term is uniquely determined by the combination of total energies of X and Y impurities in A metal with a total energy of A metal (per atom), all of which are accurately calculated by the full-potential KKR program combined with the generalized gradient approximation in the density functional formalism. We show that the magnetic energy of Ni2MnAl is reproduced very well by the present CE, from a dilute limit, including up to the 4-body interaction energies of Mn and Al impurities in Ni-bcc metal.