CCSD does not reach gauge invariance at the CBS limit, as shown in Figure (c), with a difference of ∆ TQ/LG(OI) U between LG(OI) and MVG equal to 0.26 (i.e., 26%). This indicates that the treatment of electron correlation for this property is not quite at convergence for CCSD, at least for this test set. It would be interesting to see what level of CC expansion (and basis set size) is necessary to reach gauge invariance for these molecules, but these calculations are currently beyond our capabilities. While gauge invariance for the systematically improvable CC methods would correspond to the exact OR values for a molecule (at least in terms of electronic response), gauge invariance provides no such guarantee for approximate density functionals. This is clearly shown in Figure (a), where B3LYP and CAM-B3LYP do not reach the same CBS limit even if they individually reach gauge invariance. A known limitation of current density functionals is that a systematic improvement of electron correlation for a particular functional is not possible, although Autschbach and coworkers have shown that range-separated functionals can provide better optical activity results when the rangeseparation parameter is optimized to reduce the delocalization error. An important question is whether these methods provide OR estimates that become closer to the experimental results as the basis set is increased. This is a difficult question to answer because it is hard to perform a clean comparison with experimental data. In fact, solvation effects, signal averaging over multiple conformers of the same enantiomer, and vibrational contributions to the OR are all factors that significantly affect the final measured value of this property. Theoretical models to account for these effects are still rather approximate and would cloud the comparison. Therefore, we only attempted a limited comparison by focusing on 5 rigid molecules with a single stable conformer for which experimental data in gas phase are available. This comparison still suffers from the lack of vibrational effects in the calculations. With these caveats in mind, the results in Figure indicate that CAM-B3LYP is more accurate than B3LYP for every basis set/gauge choice and the aug-cc-pVTZ basis set is sufficient to obtain good agreement with experiment. The long-range part 77 of CAM-B3LYP allows a more accurate modeling of the outermost parts of the wavefunction, which is especially important for OR calculations.