Figure 6

a) Plot of experimental versus predicted values of enthalpy of formation of gas phase (kJ/mol) using the independent validation set. b) Density plot of the differences between the observed values and the predicted values using the independent validation set. The structure of the compounds with most extreme prediction errors are indicated, the positive errors correspond to compounds with triple bonds (hexa-2,4-diyne and hex-1-ynylbenzene) and the negative errors correspond to compounds with more than one cycle (coronene and bicyclo[4.4.1]undeca-2,4,7,9-tetraene).