timate of their accuracy, and these are presented in figure . At the CBS(2,3) level, we compare these to the T CutP N O = 10 -8 results, and see that CPS(6,7) is indeed closer than TightPNO as expected (MADs of 0.16 and 0.32 kcal/mol, respectively), while CPS (5,6) has an MAD of 0.59 kcal/mol. The MDs also get closer to zero, with the MD of CPS(6,7) having a very slightly negative MD due to cases like Hc RE2 and the HKMT-2 barrier heights where CPS(6,7) slightly overshoots the T CutP N O = 10 -8 energy (as seen in figure ). There are also reductions in the largest deviations by magnitude. For CPS (5,6), the largest deviation is 1.66 kcal/mol, with two more deviations also larger than the generally accepted 1 kcal/mol chemical accuracy limit for REs and BHs; for TightPNO and CPS(6,7) the largest deviations are -0.89 kcal/mol and 0.45 kcal/mol, respectively. For CBS (3,4), comparison of CPS (5,6) to TightPNO shows that the lower-level extrapolation can replicate standard TightPNO results with an MAD of 0.37 kcal/mol at a significantly reduced cost. In our development of the MME55 benchmark set we have already noted that CPS (5,6) deviates significantly (1.92 kcal/mol) from TightPNO for RE2 of the copper-dependent enzyme Hc due to issues with the specific geometry formed in this step; across the rest of the test set used here all deviations are below 0.8 kcal/mol and the MAD is 0.22 kcal/mol, so for most larger systems where standard TightPNO is unfeasible, CPS(5,6) can therefore be considered a possible alternative. The MAD of CPS(5,6) increases to 0.62 kcal/mol when compared to the more accurate CPS(6,7) results, while TightPNO itself has a MAD of 0.30 kcal/mol. These are also inflated slightly by the Hc RE2 deviations being more than twice that of the second largest deviation (3.37 kcal/mol followed by 1.07 kcal/mol for CPS (5,6), and 1.45 kcal/mol followed by -0.63 kcal/mol for TightPNO). Given that CPS(6,7) also has its own level of error, we expect that a comparison to tightened T CutP N O results at the CBS(3,4) level would give slightly higher MADs for CPS (5,6) and standard TightPNO, but they are unlikely to be above the 1 kcal/mol chemical accuracy limit for REs and BHs, particularly for the systems other than Hc.