Graaf et al. stumbled in the same "best orbitals" dilemma, and they used the CASPT2 energy dependence with the level shift as a measurement of orbital quality, 8 that is, the appropriate weight is the one that gives more stable results with respect to variations in the level shift (and thus with less intruder states problems). Although at first this may appear as a completely distinct approach, we understand it is related to our criterion. Considering that intruder states come from the reference wave function interaction with the first-order interacting space (FOIS), 54 a 3d 9occupancy with optimal orbitals may have minimal interaction with the FOIS, at the cost of larger interaction for states dominated by the 3d 10 configurations. With the orbitals becoming closer to an intermediate between optimal for d 9 and d 10 , the effect over the d 10 configurations decreases, whereas this effect increases over the d 9 configuration, until the total effect over both configurations is minimal. Furthermore, if the recovered correlation energies are almost the same for all calculated states, the states relative energies should not drastically vary with the perturbative approach. As a consequence, for this set of reference states the presence of intruder states should be small, despite of the level shift. Therefore, both criteria to measure the orbitals quality give similar results.