Neutral and cationic free-space oxygen-silicon clusters SiOn (1 < n <= 6), and possible relevance to crystals of SiO2 under pressure

Motivated by the theoretical study of Saito and Ono (2011) on three crystalline forms of SiO2 under pressure, quantum-chemical calculations on various free-space clusters of SiOn and GeOn for 1 < n <= 6 are reported here. Both neutral and cationic clusters have been examined, for both geometry and equilibrium bond lengths. Coupled clusters and correlation-corrected MP2 calculations are presented. For the cations, we emphasize especially the structural distortions occurring in removing degeneracies.