Can quantum chemistry achieve the needed accuracy? The deviations between high-accuracy experimental values of enthalpies of formation of small molecules and radicals from the Active Thermochemical Tables and computations using various theoretical methods are shown in Fig. . Each tick on the x-axes is 10 kcal/mole. All the calculations shown use atom energy corrections (AEC) to bring the numbers from the quantum chemistry calculations onto the conventional elements-in-theirstandard-state heat of formation scale. The mean absolute errors (MAE) and root mean square errors (RMSE) are computed 3 ways: (1) before applying fitted Bond-Additivity Corrections (BAC), (2) after BAC but showing training errors (no test set), and (3) after BAC using leave-one-out cross-validation. The cross-validation method gives the best estimate of the errors in predicting enthalpies of formation of most molecules. The "Before BAC" errors provide estimates of expected errors in predictions for systems with bond types absent from the training data.