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Density functional calculations are performed to investigate the electronic and structural properties of M@Si(6) (M = Pr, Gd, Ho). Generalized Koopmans' theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES). Subsequently, a detailed comparison between previously published experimental PES spectra and the present theoretical simulations helps to identify the ground state structures. The most stable M@Si(6)(-) (M = Pr, Gd, Ho) is a three-dimensional structure with the lanthanide-metal atom sitting on top (or bottom) of the regular pentagonal bipyramid. Instead, the neutral species show the impurity as a four-coordinate atom in the equatorial plane of pentagonal bipyramid. Moreover, the nucleus independent chemical shift (NICS) is discussed at various points at and over or under the center of the Si(5) ring plane, and the negative NICS values support the aromatic character of the M@Si(6)(-) (M = Pr, Gd, Ho) clusters. (C) 2011 Elsevier B.V. All rights reserved.