In addition to encompassing a diverse range of chemical features, the species chosen all have very precisely known heats of formation (Figure ). Nearly all species have an uncertainty (95% confidence interval based on ATcT) of 1 kJ/mol (0.25 kcal/mol) or less, with the exception of a few sulfur containing species (Figure ). The average uncertainty is 0.63 kJ/mol (0.15 kcal/mol), and the largest uncertainty is 2.3 kJ/mol (0.55 kcal/mol) for tetramethylthiourea, a larger sulfur containing species. Therefore, by convention, all the species have gold-standard (uncertainty < 1 kcal/mol) thermochemistry.