Figure : DFT interaction energies for large vdW complexes, in comparison to LNO-CCSD(T)/CBS benchmarks (orange horizontal lines) and FN-DMC benchmarks (blue horizontal lines). Shaded regions in orange and blue indicate the uncertainty associated with either benchmark, and dashed lines indicate the average of the two benchmarks. DFT calculations were performed using aug-cc-pVTZ for XDM and def2-QZVPD for other methods, except that def2-TZVPD was used for C 60 @[6]CPPA. All DFT interaction energies are CP-corrected. Data points and functional names are color-coded according to the manner in which dispersion is described. Where certain DFT values are missing (for M06-2X+D3 and for several DFT+XDM methods applied to C 60 @[6]CPPA), it means that E int does not appear on the scale that is shown. sources of error. Those errors can be estimated, however, and the discrepancies with respect to FN-DMC interaction energies lie outside of the combined error bars of the two calculations. As such, we will not compare DFT calculations directly to one of these benchmarks or the other, but will instead consider both benchmarks side-by-side. We do this in Fig. , comparing both benchmarks (with estimated error bars) to a variety of DFT results, which do nothing to elucidate the nature of the discrepancy between the benchmarks, as the DFT values vary widely and do not seem to hew any more closely to one set of benchmarks as compared the other. Moreover, errors are much larger than the ∼ 0.5 kcal/mol accuracy that has been documented for small, charge-neutral dimers. Whereas dispersion-inclusive DFT methods systematically (albeit slightly) overbind small dimers, as compared to CCSD(T)/CBS calculations, no such trend is observed for these larger systems and the errors seem equally likely to have either sign. This is more apparent from Fig. , which also includes baseline results for functionals that do not describe dispersion at all: B97, BLYP, PBE, PBE0, and B3LYP. As expected, these nodispersion functionals systematically underbind these complexes, by ≥ 10 kcal/mol in every case except for the phenylalanine trimer that contains multiple hydrogen bonds. In other cases, these baseline functionals predict that the complex is unbound. We next focus on two systems for which the FN-DMC and LNO-CCSD(T)/CBS benchmarks are statistically equivalent within error bars: adenine-on-circumcoronene (C3A) and the guaninecytosine tetramer (GCGC). In both cases, we find that most of the dispersion-inclusive and dispersion-corrected DFT methods exhibit absolute errors larger than 1 kcal/mol, and many that are larger than 2 kcal/mol, and which do not appear to skew towards over-or underbinding in any systematic way. This is interesting given the rather different nature of these two complexes, with hydrogen bonds present in GCGC that are absent in C3A. In the dispersion-dominated octadecane dimer and in the hydrogen-bonded phenylalanine trimer, for which the two benchmarks also agree within error bars, we find that most DFT methods overestimate the interaction energy for the former but mostly underestimate it for the latter. These errors are also larger than 1 kcal/mol. Of the larger complexes in Fig. , the only one for which the DFT errors are consistently smaller than 1 kcal/mol is the guanine trimer, but it has the smallest interaction energy at about 2 kcal/mol.