Furthermore, the CCCBD data shows a similar behavior for the differences between PBE and HSE06 in the reaction 2O → O 2 as well as for 2N → N 2 . On the other hand, for the reaction CH 4 + 1 2 O 2 → CH 3 OH, this is not the case, and the difference in reaction energy between PBE and HSE06 is only around 6 kJ/mol for large basis set sizes. We therefore conclude that the large differences between PBE and HSE06 observed in this work are not artifacts of our calculations but inherent to DFT. For all cases, HSE06 can be expected to be closer to higher-level calculations and experiments based on the data found in the CCCBD and previous studies by Liu and Kulik.