A systematic search for minimum structures of small gold clusters Aun (n=2-20) and their electronic properties

A systematic search for global and energetically low-lying minimum structures of neutral gold clusters Au-n (n=2-20) is performed within a seeded genetic algorithm technique using density functional theory together with a relativistic pseudopotential. Choosing the energetically lowest lying structures we obtain electronic properties by applying a larger basis set within an energy-consistent relativistic small-core pseudopotential approach. The possibility of extrapolating these properties to the bulk limit for such small cluster sizes is discussed. In contrast to previous calculations on cesium clusters [B. Assadollahzadeh , Phys. Rev. B 78, 245423 (2008)] we find a rather slow convergence of any of the properties toward the bulk limit. As a result, we cannot predict the onset of metallic character with increasing cluster size, and much larger clusters need to be considered to obtain any useful information about the bulk limit. Our calculated properties show a large odd-even cluster size oscillation in agreement, for example, with experimental ionization potentials and electron affinities. For the calculated polarizabilities we find a clear transition to lower values at Au-14, the first cluster size where the predicted global minimum clearly shows a compact three-dimensional (3D) structure. Hence, the measurement of cluster polarizabilities is ideal to identify the 2D -> 3D transition at low temperatures for gold. Our genetic algorithm confirms the pyramidal structure for Au-20.