The low-temperature phase diagram of bulk Co-Pt is studied with a high-throughput ab-initio method. Global, hcp-, and fcc-restricted convex hulls are constructed to evaluate stable and metastable phases. It is found that fcc-L1 is energetically degenerate with hcp-B19. Both structures are unstable with respect to phase decomposition into hcp-D0 fcc-β at low temperature. Furthermore, L1 is an adaptive structure on the fcc-restricted convex hull which relates to the low energies of antiphase boundaries. Fcc-L1 is energetically degenerate with fcc-D0 for both Co Pt and CoPt . L1 -Co Pt and L1 -CoPt belong to the fcc-restricted convex hull. They might stabilize above the Co hcp/fcc transition and remain kinetically frozen below. L1 -CoPt is energetically well above the convex hull. Its experimental observation may result from yet unexplained finite-temperature effects. © 2011 American Institute of Physics. 0 19 2 0 2 23 3 3 2 3 0 2 3