For the present study, we decided to forego explicit mapping of the DE via the curvature of the energy as a function of the electron number. Based on previous studies of the DE in f-element complexes where the DE was quantified in this manner, it is safe to assume that the non-hybrid PBE functional produces a strong DE and too much LM donation for PuO 2 . Likewise, as mentioned already, it is safe to assume that CASSCF with attainable active spaces, HF calculations, and DFT calculations with functionals that have large fractions of eX produce an over-localization of the electronic structure and too little LM donation. A reasonable extent of donation should be obtained with hybrid functionals that contain between 20-50% eX, and-for actinide systems-in particular with PBE0 (25% eX). The series of calculations shows clearly that delocalization modulated by the DE drives the extent of LM donation in PuO 2 in DFT and MC-WFT calculations as far as the 5f shell is concerned, with a pronounced decrease of the 5f occupation as determined by NPA when the fraction of eX in the functional is systematically increased. Interestingly, the 5f' occupation provides a partial counter-balance to this trend because it increases when going from 0% to 100% eX. This finding supports the interpretation of the 5f' occupation as being associated with the diffuse regions of the 5f shell as well as the donor orbitals. When the localization error of high-eX functionals takes over the description of the system, the donor orbitals are no longer sufficiently delocalized for the NBO algorithms to assign them as donation into 5f proper. This interpretation also goes along with the relatively small radial extent of the 5f shell, compared to 6d or 7s, which renders the overlap between Pu 5f and ligand orbitals near-borderline effective for covalent bonding. It is likely for this reason that the 5f occupation is the most sensitive to the DE as revealed by the present calculations.