For the first part of our analysis, we use the 10 REs and 4 BHs associated with the enzyme model reactions shown in figure as a small test set to compare CPS (5,6) and CPS (6,7) to TightPNO as alternatives for calculating reference values for DFT benchmarking. We note that the loosened values of T CutP N O (10 -5 and 10 -6 ) should not be used outside the CPS extrapolation scheme as they compromise the accuracy of TightPNO significantly. Similarly, CBS(2,3) results are unreliable due to the poor accuracy of the double-ζ basis set negatively affecting the extrapolation. While we do not consider these lowerlevel approaches as potential benchmark strategies in the subsequent DFA tests, they are useful for observing trends across the REs and BHs with the T CutP N O values, and the CBS(2,3) results allow us to compare CPS (6,7) to the tightened value of T CutP N O = 10 -8 , which could not be done for CBS (3,4) as the def2-QZVPP calculations were computationally too expen-sive. We also note that the CPS (5,6) and TightPNO results at the CBS (3,4) level for the metalloenzymes CDO, Hc and NiSOD have been taken from the brief tests we conducted when selecting the reference values for our new MME55 set, where CPS (5,6) was referred to as "estimated TightPNO".