BOND-ENERGY CALCULATIONS OF CU2, AG2, AND CUAG WITH THE GENERALIZED GRADIENT APPROXIMATION

We have calculated the bond lengths and bond energies of Cu2, Ag2, and CuAg using nonlocal-density-functional theory with the generalized gradient approximation (GGA), in order to investigate the validity of GGA to metal molecules. In GGA, an exchange-correlation energy density is approximated by a function of electron density rho(r), and its coordinate differences partial-rho(r), which are not considered in the local-spin-density approximation (LSDA). We performed fully self-consistent calculations, considering all electrons: the molecular orbitals were expanded by a Slater-type atomic basis, and a nonrelativistic approximation was used. The calculated bond energies of Cu2, Ag2, and CuAg in GGA are 0.4 eV (15%), 0.34 eV (17%), and 0.38 eV (17%) smaller than those results in LSDA, so that GGA removes about 2/3 of the overestimated value of bond energies in LSDA.