Across the singlet REs of CDO, NiSOD and Hc, the estimated (T 1 ) approach is the best alternative to DLPNO-CCSD(T 1 )/TightPNO/CBS (3,4), with deviations ranging from 0 kcal/mol (CDO 1RE1) to -0.497 kcal/mol (Hc 1RE1). Estimated CBS(3,4) also gives good results with a more significant cost reductionnone of the deviations are larger than 1 kcal/mol, the generally considered "chemical accuracy limit" for REs and BHs. For both of these approaches, we have also tested double-ζ and CBS(2,3) correction terms (table S2 in the Supporting Information), but both perform worse than the triple-ζ-based corrections. This includes CBS (3,4) with CBS(2,3) triples, which is similar to a Weizmann-1 extrapolation -it has a mean absolute deviation (MAD) of 0.744 kcal/mol, more than three times that of the triple-ζ-corrected version shown here.