Using the MME55 set, we have tested the density functionals listed in Level of theory: a DLPNO-CCSD(T 1 )/CBS, b estimated (T 1 ) (see eq. 4), c estimated CBS(3,4) (see eq. 5), d estimated TightPNO (uses E Corr values calculated following eq. 6 for the CBS extrapolation). e A number in front the label indicates multiplicity when needed for clarity. f Barrier of reverse reaction. g Specific details of each step can be found in the Supporting Information.