Electronic structure and bonding in anionic coinage metal clusters are investigated via density-functional calculations, focusing on an extensive set of isomers of ${\mathrm{Cu}}_7^{-}$, ${\mathrm{Ag}}_7^{-}$, and ${\mathrm{Au}}_7^{-}$. While the ground states of ${\mathrm{Cu}}_7^{-}$ and ${\mathrm{Ag}}_7^{-}$ are three dimensional (3D), that of ${\mathrm{Au}}_7^{-}$ is planar, separated from the optimal 3D isomer by 0.5 eV. The simulated thermally weighted photoabsorption spectrum of ${\mathrm{Au}}_7^{-}$ is dominated by planar structures, and it agrees well with the measured one. The propensity of ${\mathrm{Au}}_N^{-}$ 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.

• Received 3 January 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.033401