Understanding the structural transformation, stability of medium-sized neutral and charged silicon clusters

The structural and electronic properties for the global minimum structures of medium-sized neutral, anionic and cationic Si-n(mu) (n = 20-30, mu = 0, -1 and + 1) clusters have been studied using an unbiased CALYPSO structure searching method in conjunction with first-principles calculations. A large number of low-lying isomers are optimized at the B3PW91/6-311 + G* level of theory. Harmonic vibrational analysis has been performed to assure that the optimized geometries are stable. The growth behaviors clearly indicate that a structural transition from the prolate to spherical-like geometries occurs at n = 26 for neutral silicon clusters, n = 27 for anions and n = 25 for cations. These results are in good agreement with the available experimental and theoretical predicted findings. In addition, no significant structural differences are observed between the neutral and cation charged silicon clusters with n = 20-24, both of them favor prolate structures. The HOMO-LUMO gaps and vertical ionization potential patterns indicate that Si-22 is the most chemical stable cluster, and its dynamical stability is deeply discussed by the vibrational spectra calculations.