Structures of [Ag7(SR)4]- and [Ag7(DMSA)4]-.

We have developed a new genetic algorithm approach to search for the global lowest-energy structures of ligand-protected metal clusters. In combination with density functional theory, our genetic algorithm simulations show that the ground state of [Ag(7)(DMSA)(4)](-) has eight instead of four Ag-S bonds and has a much lower energy than the structure based on the [Ag(7)(SR)(4)](-) cluster with a quasi-two-dimensional Ag(7) core. The simulated X-ray diffraction pattern of the [Ag(7)(DMSA)(4)](-) cluster is in good agreement with the experimental result. Our calculations for the [Ag(7)(SR)(4)](-) and [Ag(7)(DMSA)(4)](-) clusters reveal for the first time that -RS-Ag-RS- can be a stable motif in thiolate-protected Ag clusters. In addition, the lowest-energy structures of [Ag(7)S(4)](-), [Ag(6)S(4)](-), and [Ag(5)S(4)](-) are predicted.