Structure and stability of Na6Pb clusters

The geometric and electronic structures of the bimetallic cluster, Na6Pb, h ave been calculated using the density functional and coupled-cluster approa ches, with a relativistic effective core potential and one-electron spin-or bit operators. Six possible isomeric Na6Pb structures (O-h, D-3h, D-3d, C-5 upsilon, C-3 upsilon, and C-2 upsilon) have been examined, and the highly symmetrical O-h structure was found to be the most stable form. The stabili ties of the clusters correlate well with the Pb-Na bond distances and with the charge transfer from the Na-6 subsystem to the more electronegative Pb atom. The binding energies were found to vary according to the structure, w ith a spread of about 0.1 eV and 0.3 eV for the density functional and coup led-cluster calculations, respectively. These are of the same order, and so mewhat smaller, as are found for the Na6Mg clusters (about 0.5 eV), demonst rating that the stabilization of the Na6Pb cluster is not particularly depe ndent on the geometric structures. At the same time, we found that the Na6P b cluster is more stable as the symmetry is higher. (C) 2000 American Insti tute of Physics. [S0021-9606(00)31140-0].