The natural orbitals obtained from the state-averaged CASSCF(4,7) calculations and the associated occupation numbers for the scalar relativistic ground spin-quintet state are displayed in Figure . The four unpaired electrons are predominantly distributed over the 5f t2u and 5f t1u orbitals, with aggregate occupations of 2.74 and 1.17, respectively, whereas the rather strongly σ-antibonding 5f a2u orbital has a population of 0.08. (The corresponding bonding orbital was inactive in the present calculations and doubly occupied.) As it was already demonstrated in Table and Figure of Reference 6, and therefore not repeated here by new calculations, when the SO interaction is reflected in the ground-state wavefunction the occupations become more evenly distributed over the 5f subshells, which is a consequence of excited SF spin-states mixing into the ground spin state. The PuO 2 SF ground spin state deriving from the 5 I ion term contributes the dominant weight (ca. 0.5) in the SO ground state wavefunction and is therefore suitable for investigating the extent of LM donation in the system. The following discussion is based on the analysis of the SF ground spin state of PuO 2 as obtained from different calculations.