The test performance of the GNN model for gas-phase acidities on held-out QM data can be seen in Figure . The ratio of acceptable error to the range of property values is quite low. Gas-phase acidities span roughly 100 kcal mol -1 , but values accurate enough for room-temperature kinetic predictions should have errors less than 2 kcal mol -1 , which is lower than the aleatoric uncertainty of experimental gas-phase acidity data, i.e. we desire to predict gas-phase acidities with <2% error. Hence, although the error metrics are somewhat high for the gas-phase acidities (the RMSEs are around 4 kcal mol -1 , twice as large as tolerable error), the percent errors of about 4% are fairly low, and the model generally captures the overall trends and relative acidities among different compounds.