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).
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Abstract. Mechanical vibration with different frequencies was operated during the direct laser metal deposition process in order to discuss the effects of vibration on residual stress and other characters of deposited workpiece in this study. Sub-resonance, resonance and the optimum frequency (OPF) were selected for operation. The results revealed that the residual stress of the deposited workpiece was a tensile stress. With the operation of mechanical vibration, the stress of clad workpiece decreased, and the workpiece vibrated with the OPF showed the lowest residual stress. Microstructures of all the specimens with vibration refined, and the finest structure appeared in the specimen vibrated with the OPF. The hardness result showed that the lower the residual stress was, the lower the hardness was, and a low residual stress and hardness would have advantage of subsequent process of the deposited workpiece.
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