Often in DFT calculations, quadruple-ζ energies are taken to be equivalent to the CBS limit. Except for double-hybrid functionals, we will define the DFT/ CBS limit using the average of CP-corrected and uncorrected values of ∆E int computed using a quadruple-ζ basis set, namely aug-cc-pVQZ for small dimers and def2ma-QZVP 88 for larger systems. For the dimers in the S66 data set, CP correction at the aug-cc-pVQZ level typically amounts to < 0.1 kcal/mol, indicating convergence to the DFT/CBS limit. For larger systems, the magnitude of the CP correction in the def2-ma-QZVP basis set affords an estimate of the uncertainty in the DFT/CBS limit. For double-hybrid functionals we estimate the CBS limit using the SCF/aug-cc-pVQZ hybrid DFT energy plus a two-point "T/Q" extrapolation of the MP2 correlation energy, where the latter is computed using aug-cc-pVTZ and the aug-cc-pVQZ. All calculations were performed using Q-Chem v. 5.4. Integral screening and shell-pair drop tolerances were both set to τ ints = 10 -12 a.u. and the self-consistent field (SCF) convergence threshold was set to τ SCF = 10 -8 Ha, except for the protein-ligand systems in Section 3 .3 .2, for which we use τ ints = 10 -10 a.u. and τ SCF = 10 -6 Ha. The SG-2 quadrature grid is used for all B97-based functionals and SG-3 for the Minnesota functionals. For other functionals, we use SG-1. 3 Results and Discussion 3.1. S66 Data Set. We will use the S66 data set to obtain a systematic understanding of the effect of CP correction on ∆E int . The largest of the S66 dimers contains 34 atoms (pentane dimer), and this small size allows us to use quadruple-ζ calculations even for computationallydemanding functionals, and thus to definitively establish the CBS limit.