55-atom clusters of silver and gold:: Symmetry breaking by relativistic effects

Anionic 55-atom clusters of gold and silver are studied using density functional theory, scalar relativistic ab initio pseudopotentials and self-consistent generalized gradient corrections. An almost perfect icosahedron is found to be the clear ground state of Ag-55(-) and its electronic density of states agrees almost perfectly with recently measured high-resolution photoelectron spectra, up to the magnitude of the splitting of the highest free-electron shells by the I-h crystal field. A comparison between theory and a recent experiment allows one to assign icosahedral-based structures also for the Ag-57(-) cluster. On the other hand, the Au-55(-) cluster has several close-lying low-symmetry isomers with a washed-out electron shell structure. This qualitative difference between silver and gold clusters is related to the strong relativistic bonding effects in gold. (c) 2005 Elsevier B.V. All rights reserved.