Mixing enthalpy of the GaAs-AlAs random alloy: 64-atom supercell calculations

We have calculated the formation enthalpy of two ${\mathrm{Ga}}_{16}$ ${\mathrm{Al}}_{16}$ ${\mathrm{As}}_{32}$ alloys where the Ga and Al are randomly distributed on one of the fcc sublattices in a 2a×2a×2a cubic unit cell. In disagreement with our average t-matrix calculation, which found a negative enthalpy for the random alloy, but in agreement with the atomistic calculation of Wei and Zunger, we find that the random alloy has a formation enthalpy somewhat less positive than the (GaAs${)}_1$(AlAs${)}_1$(001) superlattice.