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 hight-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 sturcture. This qualitative difference between silver and gold clusters is related to the strong relativistic bonding effects in gold.