We explore the structural evolution of (Au 2 S) n (n ¼ 1-8) clusters using a first principles global minimization technique, namely, a genetic algorithm from density functional theory geometry optimization (GA-DFT).The growth sequence and pattern for n from 1 to 8 are analyzed from the perspective of geometric shell formation. The average binding energy, HOMO-LUMO energy gaps, vertical electron affinity, and vertical ionization potential are examined as a function of the cluster size. The global minimum structures are planar at n ¼ 1-3, three-dimensional at n ¼ 4-8. The formation of these structures are attributed to the high stability of S-Au-S structural unit and particularly the Au 3 S 3 and Au 4 S 4 rings. Chemical bonding analysis reveals that the three-dimensional clusters (n ¼ 4-8) can be viewed as [Au 2nÀx S n ] xÀ $xAu + in electronic structure. The Au + cations are not involved in any S-Au covalent bond, however, are attracted by only Au/Au aurophilic interactions. Direct evidence for the Au/Au aurophilicity are given by a noncovalent interaction index analysis. Such Au/Au aurophilic interactions play an important role in the stability of (Au 2 S) n clusters. † Electronic supplementary information (ESI) available: AdNDP localized bonding patterns of (Au 2 S) n (n ¼ 5-8) and atomic coordinates (inÅ) of the global minimum structures. See Fig. 1 The global minimum and low-energy isomers and the bonding framework of (Au 2 S) n (n ¼ 1-8) clusters at the TPSS/LanL2DZ/6-31G* level. Labelled are the point groups and relative energies in eV. (Au yellow, S purple). This journal is

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