As we found for SA-CASSCF, we find that tPBE (0.25 ± 0.02 eV) and tPBE0 (0.20 ± 0.02 eV) maintain relatively consistent errors across different active spaces and basis set sizes when the 1.1 eV SA-CASSCF error threshold is applied. This is an intuitive result, as the accuracy of MC-PDFT is primarily contingent on the quality of the SA-CASSCF density and on-top density and on the quality of the on-top functional; if one eliminates the poor active spaces, then the functional (correlation) error may dominate, and this is approximately independent of the active space and basis set. In contrast, NEVPT2 shows quantitatively worse results as the basis set is decreased even as the wave function remains qualitatively well-described. This makes sense because the power of NEVPT2 to change the SA-CASSCF energy stems from its perturber states, which are less capable of describing dynamic correlation within a smaller basis because the smaller basis set cannot represent the virtual-orbital space as well. Figure shows that the difference between the tPBE0 excitation energy and the SA-CASSCF excitation energy remains fairly consistent across active spaces and basis sets, but there is a significant drop in the NEVPT2 correction when moving from aug-cc-pVTZ to cc-pVDZ, resulting in increased NEVPT2 error. Figure combined with Figure shows clearly how the NEVPT2 results degrade in quality with decreasing size of the basis set, while the performance of tPBE0 remains consistent. As the basis set is decreased in size, the mean absolute change to the SA-CASSCF excitation energy decreases for NEVPT2 while remaining constant for tPBE. These results provide a plausible explanation of the discrepancy in mean absolute error found for SC-NEVPT2 between the study of Schapiro et. al. (0.23 eV) and the more recent study of Sarkar et. al. (0.15 eV). They imply that it is due to the fact that the Sarkar study used the aug-cc-pVTZ basis while the Schapiro study employed the cc-pVTZ basis. However, our results point to this being caused by a strictly poorer performance of NEVPT2 with the smaller basis set and not due to a poorer zeroth-order description of the underlying wave functions.