Bulk fragment and tubelike structures of Au_N (N=2-26)

Using the relativistic all-electron density-functional calculations on the Au_N (N=2-26) in the generalized gradient approximation, combined with the guided simulated annealing, we have found that the two- to three-dimensional structural transition for Au_N occurs between N=13 and 15, and the Au_N (16≤N≤25) prefer also the pyramid-based bulk fragment structures in addition to the Au₂₀. More importantly, the tubelike structures are found to be the most stable for Au₂₄ and Au₂₆, offering another powerful structure competitor with other isomers, e.g., amorphous, bulk fragment, and gold fullerene. The mechanism to cause these unusual Au_N may be attributed to the stronger s-d hybridization and the d-d interaction enhanced by the relativistic effects.