MC-WFT Calculations: MC-WFT calculations were performed with the complete active space self-consistent field (CASSCF) method. In a subset of these calculations, the dynamic electron correlation was treated by second-order N-electron valence state perturbation theory (NEVPT2) in its strongly contracted flavor and correlating all orbitals. Scalar relativistic effects were included variationally in the calculations via the second order Douglas-Kroll-Hess (DKH2) relativistic Hamiltonian. The spin-orbit (SO) interaction was treated via the interaction of scalar relativistic ('spin-free', SF) many-electron spin states, and the susceptibility was calculated using quasi-degenerate perturbation theory (QDPT). Unfortunately, the ORCA code does not output magnetic transition moments between individual state components, and therefore a calculation of the susceptibility from Reference 6 using only states deriving from the Pu(IV) 5 I 4 level could not be replicated in the present work. Otherwise, however, the susceptibility data of the present study and Reference 6 are in excellent agreement. Segmented all-electron relativistically contracted (SARC) basis 101 of triplezeta polarized quality was used for plutonium and the def2-TZVP 102 basis was used for oxygen. The AUTOAUX feature of ORCA was used to automatically generate corresponding auxiliary basis sets for the calculations. 103 A (4,7) active space (seven 5f orbitals with four electrons) was used for most of the MC-WFT calculations, following previous benchmarks. For a reasonable description of the SO coupling at the metal site, the mixing of a considerable number of SF electronic states is beneficial. Accordingly, CASSCF/NEVPT2+SO calculations were performed considering all possible spin states arising from the (4,7) active space, viz., 35 spin-quintets, 210 spin-triplets, and 196 spin-singlets. Sets of spin states for a given multiplicity were determined in a stateaveraged fashion. Additional calculations were performed with a (4,12) active space in which Pu-centered orbitals of 6d parentage were added.