the experimental value, with an error of 2,172 kJ mol -1 . This error arises due to the implicit assumption that the atomization energies are correctly computed with BEEF-vdW, but it is well known that much higher levels of theory are needed to accurately compute total atomization energies . The isogyric reaction gives a much better agreement with the experiment, with a deviation of only 42 kJ mol -1 , but the deviation is still large enough to cause equilibrium constants to be off by orders of magnitude. The highest possible rung in the CBH scheme for this reaction is an isodesmic reaction for 1-propyl. With this reaction, ∆ f H is determined to within chemical accuracy (3 kJ mol -1 ) of the accurate ATcT value from BEEF-vdW energies. Although the gas-phase community has highly accurate electronic structure methods, error cancellation is still a very active field of research . This example illustrates the power of error cancellation in a gas-phase context, and the same concepts hold for surface reactions, although some modifications are required.