Structural evolution of (Au2S)n (n=1-8) clusters from first principles global optimization

We explore the structural evolution of (Au2S)(n) (n = 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 = 1-3, three-dimensional at n = 4-8. The formation of these structures are attributed to the high stability of S-Au-S structural unit and particularly the Au3S3 and Au4S4 rings. Chemical bonding analysis reveals that the three-dimensional clusters (n = 4-8) can be viewed as [Au2n-xSn](x-)center dot xAu(+) in electronic structure. The Au+ cations are not involved in any S-Au covalent bond, however, are attracted by only Au center dot center dot center dot Au aurophilic interactions. Direct evidence for the Au center dot center dot center dot Au aurophilicity are given by a noncovalent interaction index analysis. Such Au/Au aurophilic interactions play an important role in the stability of (Au2S) n clusters.