Bonding in Cu, Ag, and Au Clusters: Relativistic Effects, Trends, and Surprises

Abstract

Electronic structure and bonding in anionic coinage metal clusters are investigated via density-functional calculations, focusing on an extensive set of isomers of ${\mathrm{Cu}}_{7}^{\ensuremath{-}}$, ${\mathrm{Ag}}_{7}^{\ensuremath{-}}$, and ${\mathrm{Au}}_{7}^{\ensuremath{-}}$. While the ground states of ${\mathrm{Cu}}_{7}^{\ensuremath{-}}$ and ${\mathrm{Ag}}_{7}^{\ensuremath{-}}$ are three dimensional (3D), that of ${\mathrm{Au}}_{7}^{\ensuremath{-}}$ is planar, separated from the optimal 3D isomer by 0.5 eV. The simulated thermally weighted photoabsorption spectrum of ${\mathrm{Au}}_{7}^{\ensuremath{-}}$ is dominated by planar structures, and it agrees well with the measured one. The propensity of ${\mathrm{Au}}_{N}^{\ensuremath{-}}$ clusters to favor planar structures (with $N$ as large as 13) is correlated with strong hybridization of the atomic $5d$ and $6s$ orbitals due to relativistic effects.