  The ground state equilibrium properties of copper-gold alloys have been
explored with the state of art random phase approximation (RPA). Our estimated
lattice constants agree with the experiment within a mean absolute percentage
error (MAPE) of 1.4 percent. Semi-local functionals such as the generalized
gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) and strongly
constrained and appropriately normed (SCAN) fail to provide accurate bulk
moduli, which indicate their inability to describe the system in a stretched or
compressed state with respect to the equilibrium geometry. We find that the
non-locality present in RPA is able to describe the transition between two
delocalized electron densities (bulk elemental constituents to crystallized
alloys), as required to provide accurate formation energies. Based on our
results, we conclude that it is difficult to find a universal density
functional which can give accurate results for a wide range of properties of
intermetallic alloys. However, RPA can capture different bonding situations,
often better than other density functionals. It gives accurate results for a
wide range of ground state properties for the alloys, generated from metals
with completely filled d-shells.
