We also modelled various three-oxygen species; a y-lid flanked by two BC Oi atoms, previously proposed as a thermal donor core, and hereafter referred to as the Snyder-Stavola model[249,211]. We also examined a partially dissociated structure consisting of a pair separated by two Si-Si bonds from isolated Oi. In this second structure the `inner' O of the pair can become trivalent, allowing a reconstructed Si-Si bond to form similar to the di-y-lid structure (see Figure 9.14). A third possibility is an oxygen square structure with one flanking Oi, and in practise the Snyder-Stavola model relaxes towards this with a tiny saving in energy.