Figure S8(b) shows the dependence of the number of non-bridging oxygen atoms per tetrahedron N_{NBO}/N_T on the Al-O coordination number for the tectosilicate glasses with f_{NBO} ~0 at rho^prime = 1. The fraction of NBO atoms was calculated by assuming that f_{Si(IV)} = 1 and only the Si(IV) and Al(IV) species contribute towards the network.