At 355 nm, left panels in Figure , the errors are dominated by molecule 30, which is a notoriously difficult case for simulations. The increase in basis set size does not provide a significant improvement of the results. More informative are the results at 633 nm, shown in the right panels in Figure , where the errors are more evenly distributed. Also in this case, the basis set incompleteness is not the most important source of error; this is the case also for the choice of gauge with smaller basis sets. Overall, CAM-B3LYP shows a smaller ∆ Exp U with each choice of gauge and basis set compared to B3LYP by about 0.05-0.15 units. The smallest ∆ Exp U S values are obtained for both methods with LG(OI) and the CBS(DT) extrapolation, which is likely fortuitous. From this data, it is not possible to determine whether the disagreement with experiment at the CBS limit is due to missing vibrational effects or electron correlation incompleteness in the calculations, although it is likely that both contributions are important. 5 Discussion and Conclusions