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 Ag55-, 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 Ih crystal field. A comparison between theory and a recent experiment allows one to assign icosahedral-based structures also for the Ag57- cluster. On the other hand, the Au55- 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.