Ab-initio study of the Coulomb interaction in NbxCo clusters: Strong on-site versus weak non-local screening

By means of ab-initio calculations in conjunction with the random-phase approximation (RPA) within the full-potential linearized augmented plane wave method we study the screening of the Coulomb interaction in NbxCo (1<=x<=9) clusters. In addition, these results are compared with pure bcc Nb bulk. We find that for all clusters the onsite Coulomb interaction in RPA is strongly screened whereas the inter-site non-local Coulomb interaction is weakly screened and for some clusters it is unscreened or even anti-screened. This is in strong contrast with pure Nb bulk, where the inter-site Coulomb interaction is almost completely screened. Further, constrained RPA calculations reveal that the contribution of the Co 3d ! 3d channel to the total screening of the Co 3d electrons is small. Moreover, we find that both the onsite and inter-site Coulomb interaction parameters decrease in a reasonable approximation linearly with the cluster size and for clusters having more than 20 Nb atoms a transition from 0D to 3D screening is expected to take place.