Geometric structure of silver clusters with and without adsorbed Cl and Hg

We report density functional theory calculations, with the TPSS functional, for the clusters Agn (n = 3-20), AgnCl2 (n = 3-10, and n = 12) and AgnHgm (n = 5, 6 and m = 1-4). We performed unbiased global optimization for all of these species by Tabu Search in Descriptor Space. The Agn global minima geometries that we find confirm the earlier PBE functional results of Yang et al. (M. Yang, K.A. Jackson, J. Jellinek, First-principles study of intermediate size silver clusters: shape evolution and its impact on cluster properties, J. Chem. Phys. 125 (2006) 144308 (8pp.)). We find special stability associated with Ag6, Ag7, and Ag8 which we explain by their closed electronic shell (n = 6, 8) or closed atomic shell (n = 7). The Ag16 cluster stands out in that it is particularly unstable, it has a small HOMO-LUMO gap, and it has a high-coordination binding site (the site of the missing thirteenth atom of an icosahedron). This may explain the experimental observations (L. Lian, S.A. Mitchell, P.A. Hackett, D.M. Rayner, Size dependence of mobility on the surface of metal clusters: ammonia on Ag10 and Ag16, J. Chem. Phys. 104 (1996) 5338-5344) that NH3 binds strongly to Ag16 and does not diffuse on it. We calculate the Cl-Agn binding energy to be roughly 3.0 eV in most cases, and the Hg-Agn to be roughly 0.25 eV. Binding of two Cl atoms severely disrupts the Agn geometric structure in some cases (n = 5, 6, 7), but leaves the Agn structures essentially intact in other cases (n = 4 and 9). Binding to Hg atoms has little effect on the geometry of Ag5 but a big effect on that of Ag6.