Some difference in the N2O activation energy barrier between the PBE and HSE06 functionals (ΔΔEB, see Figure for a visual comparison) can be expected due to the artificially large electron delocalization of GGA functionals that often occurs in transition metal systems. The magnitude of the difference in this case (48 kJ/mol) is, however, striking. We therefore investigated the origin of this large deviation by performing calculations for the simple gas-phase reaction N 2 + 1 2 O 2 → N 2 O (see SI for details). The reaction energy difference (ΔΔER) between PBE and HSE06 for this simple gas-phase reaction is, at around 30 kJ/mol, comparable and surprisingly large for such a simple reaction. Our own findings are supported by a larger set of reference calculations in the Computational Chemistry Comparison and Benchmark Database (CCCBD) by NIST. We find good agreement between our calculated ΔΔER between HSE06 and PBE in the CCCBD for the reaction N 2 + 1 2 O 2 → N 2 O. Based on the CCSD(T)=FULL reaction energy from the CCCBD, we argue that the HSE06 calculations should be deemed more trustworthy, as they are much closer to the high-level reference than the PBE results.