Such results tend to be underestimates of the density, possibly owing to the use of PBE-sol and the lack of van der Waals interactions. However, any such underestimation of the density and lattice constants was within an acceptable error range. The errors of 12% and -8.6% for the density for zircon and α-Al The system density is a fundamental property of glass that has been used to evaluate the accuracy of the potential parameters for CMD. Figure In the composition range greater than 33 mol%, the density derived from NNMD was in poorer agreement with the experimental data than that from CMD-Du; this appeared to be owing to error related to PBE-sol. An expensive exchange-correlation functional (the first meta-generalized-gradient approximation) was used to generate the training data; this led to a SiO 2 glass density of 2.16 g/cm 3 (experimental value: 2.20 g/cm 3 ). Bertani et al. also argued that the simulated density predicted from NNMD was influenced by the type of exchange-correlation functional used to generate the training data. In addition, an unrealistic fast cooling rate in NNMD induces an incorrect spatial arrangement of Na-Na. The latter is supported by the mismatch of the Li-Li partial structure factor found in NNMD studies of lithium borosilicate glasses. Improvements in accuracy may be achieved by constructing more abundant and varied training data for Na-Na configurations. The silicate glass network structure was characterized in terms of Q n structure, where n is the number of bridging oxygens bonded to Si. Figure shows the populations of Q n as a function of Na 2 O content.