The full story of the Si 6 magic cluster

Abstract

The structural, electronic, dynamical and spectral properties of Si₆ and its ions ( ${\mathrm{Si}}_6^{1-}$ , ${\mathrm{Si}}_6^{2-}$ , and ${\mathrm{Si}}_6^{1+}\mathrm{)}$ have been examined using a variety of high level ab initio techniques, including quadratic configuration interaction, coupled cluster, and density functional theory (DFT) with the hybrid B3LYP functional. Various high quality correlation-consistent basis sets, ranging from 2Z up to 5Z quality, were employed for the DFT calculations. It is shown that not only the ground state structure, but also the structure of the excited states, as well as the structure of the anions and cations of Si₆ are controversial. Each one of the three competing structures for the ground state, with C_s/C_2v, D_4h, and C_2v symmetry, has been considered by different investigators as the lowest energy structure either of the neutral cluster, or of its anion or cation (or both). In a spirit of “structural democracy” it is demonstrated that the C_s/C_2v, D_4h, and _2v structures can be safely assigned as the ground states of the neutral, anion, and cation clusters respectively. The present results, which support the structural plasticity (fluxionality) of Si₆, are in excellent agreement with experiment, including Raman and IR spectra, ionization energies, electron affinities as well as vibrationally resolved photoelectron spectra. The paradigm of Si₆ could be very helpful for other silicon clusters as well.