A new approach which involves a diffuse multi-layer fault model coupled with a cluster approach to statistical mechanics (CASM) infrastructure has been developed to identify the proximate structure for the antiphase boundary (APB) on {111} planes in L12. We report a new structure, ‘γa′' which approximates the bonding environment of an APB better than a previously reported structure, ‘ω'. Density functional theory calculations were employed to predict the APB energies from structural energies of γa′ and L12 and were validated against estimations from supercell methods for several binary compounds. The implications of the discovery of the γa′ are discussed.