Method and Results

We used a 130 atom cluster surrounding a vacancy to which O atoms were successively added, Si₇₀H₆₀O$\rm_n$. All the atoms other than the surface H atoms were relaxed, and symmetry was constrained where appropriate. Unless stated otherwise the defects were all charge neutral and spin averaged. For many of the later runs we experimented with many of the cluster parameters, e.g. the status of the surface H atoms (fixed, free, or constrained in quadratic `spring' potentials), and the arrangement of additional bond centred fitting functions. These are discussed below in the relevant sections.