Figure 17(a) shows the dependence of the fraction of non-bridging oxygen atoms f_{NBO} on the reduced density rho^prime for the depolymerized glasses with f_{NBO} ~0.22-0.24 at rho^prime = 1. The fraction of NBO atoms was calculated by assuming that f_{Si(IV)} = 1 and all the Si(IV) and aluminum species contribute towards the network.