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Ionization of medium-sized silicon clusters and the geometries of the cations

We have performed a systematic ground state geometry search for the singly charged Sin cations in the medium-size range (n⩽20) using density functional theory in the local density approximation (LDA) and generalized gradient approximation (GGA). The structures resulting for n⩽18 generally follow the prolate “stacked Si9 tricapped trigonal prism” pattern recently established for the lowest energy geometries of neutral silicon clusters in this size range. However, the global minima of Sin and Sin+ for n=6, 8, 11, 12, and 13 differ significantly in their details. For Si19 and Si20 neutrals and cations, GGA renders the prolate stacks practically isoenergetic with the near-spherical structures that are global minima in LDA. The mobilities in He gas evaluated for all lowest energy Sin+ geometries using the trajectory method agree with the experiment, except for n=18 where the second lowest isomer fits the measurements. The effect of gradient corrections for either the neutral or cationic clusters is subtle, but...