Building blocks and structural patterns in silicon clusters : global and local optimizations employing empirical potentials, density functionals, and ab-initio calculations
Sin clusters in the size range n=4-35 have been investigated, using a combination of global structure optimization methods with DFT and ab-initio calculations. One of the central aims is to provide explanations for the structural transition from prolate to spherical outer shapes at about n=25, as observed in ion mobility measurements. First, several empirical potentials for silicon and a newly generated variant of one of them were better adapted to small silicon clusters, by global optimization of their parameters. The best resulting empirical potentials were then employed in global cluster structure optimizations. The most promising structures from this stage were relaxed further at the DFT level with a hybrid B3LYP functional. For the resulting structures, single point energies have been calculated at the LMP2 level with cc-pVTZ basis set. Results obtained at the DFT level strongly support the shape transition form prolate to spherical structures beginning with Si26. In contrast, at the LMP2 level, the dominance of spherical structures after the transition region could not be confirmed.
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