COHESIVE ENERGY OF 3D TRANSITION-METALS - DENSITY-FUNCTIONAL THEORY ATOMIC AND BULK CALCULATIONS

We report generalized gradient approximation (GGA) cohesive energies f or 3d metals. The problem of obtaining atomic reference energies in de nsity-functional theory is considered. The effect of going to nonspher ical atomic charge distributions is much larger at the GGA than at the local-density approximation (LDA) level, but allowing fractional occu pations of 3d and 4s shells has negligible effect. When nonsphericity effects are taken into account in the atomic reference energies, the a verage absolute error of 0.3 eV in the GGA cohesive energies is much s maller than the LDA error of 1.3 eV. The working of the GGA is analyze d in terms of the cohesive energy density and the charge inhomogeneity division of the exchange energy. The low-gradient limit in the GGA fu nctionals is not important as regions with charge inhomogeneity s<0.2 have negligible contributions.