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Chiral structures have been found as the lowest-energy isomers of bare (Au 28 and Au 55 ) and thiol-passivated ͓Au 28 (SCH 3 ) 16 and Au 38 (SCH 3 ) 24 ] gold nanoclusters. The degree of chirality existing in the chiral clusters was calculated using the Hausdorff chirality measure. We found that the index of chirality is higher in the passivated clusters and decreases with the cluster size. These results are consistent with the observed chiroptical activity recently reported for glutahione-passivated gold nanoclusters, and provide theoretical support for the existence of chirality in these compounds.Detailed knowledge of the lattice structure, shape, morphology, surface structure, and bonding of bare and passivated gold clusters is fundamental to predict and understand their electronic, optical, and other physical and chemical properties. This information is essential to optimizing their utilization as novel nanocatalysts, 1 and as building-blocks of new molecular nanostructured materials, 2 with potential applications in nanoelectronics 3 and biological diagnostics. 4 An effective theoretical approach to determine gold cluster structures is to combine genetic algorithms and many-body potentials ͑to perform global structural optimizations͒, and first-principles density functional theory ͑to confirm the energy ordering of the local minima͒. Using this procedure we recently found many topologically interesting disordered gold nanoclusters with energy near or below the lowestenergy ordered isomer. 5-8 The structures of these clusters showed low spatial symmetry or no symmetry at all, opening the possibility of having distinct electronic and optical properties in such systems. In other studies on passivated gold nanoclusters, 9,10 we also found that the effect of a methylthiol monolayer ͑24 SCH 3 molecules͒ on a truncatedoctahedron ͑with fcc geometry͒ Au 38 cluster is strong enough to produce a dramatic distortion on the gold cluster, resulting in a disordered geometry for the most stable Au 38 (SCH 3 ) 24 passivated cluster.Although the calculated structure factors of the disordered gold clusters were in qualitative agreement with the data obtained from x-ray powder diffraction on experimental samples, 6,7 the direct confirmation of the existence of bare and thiol-passivated gold nanoclusters with low or no spatial symmetry had not been possible due to the lack of enough experimental resolution for clusters in the size range of 1-2 nm. 11-13 Nevertheless, in a recent study using circular dichroism, Shaaff and Whetten ͑SW͒ 14 found a strong optical activity in the metal-based electronic transitions ͑across the near-infrared, visible and near ultraviolet regions͒ of sizeseparated glutathione-passivated gold clusters in the size range of 20-40 Au atoms. SW pointed out that the most plausible interpretation of these results is that the structure of the metal-cluster core of the gold-glutathione cluster compounds would be inherently chiral. Moreover, since the most abundant cluster in the experimental samples corresponds...

Knowledge of the structure of clusters is essential to predict many of their physical and chemical properties. Using a many-body semiempirical Gupta potential ͑to perform global minimizations͒, and first-principles density functional calculations ͑to confirm the energy ordering of the local minima͒, we have recently found ͓Phys. Rev. Lett. 81, 1600 ͑1998͔͒ that there are many intermediate-size disordered gold nanoclusters with energy near or below the lowest-energy ordered structure. This is especially surprising because we studied ''magic'' cluster sizes, for which very compact-ordered structures exist. Here, we show how the analysis of the local stress can be used to understand the physical origin of this amorphization. We find that the compact ordered structures, which are very stable for pair potentials, are destabilized by the tendency of metallic bonds to contract at the surface, because of the decreased coordination. The amorphization is also favored by the relatively low energy associated to bondlength and coordination disorder in metals. Although these are very general properties of metallic bonding, we find that they are especially important in the case of gold, and we predict some general trends in the tendency of metallic clusters towards amorphous structures.

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