Ligand-Field DFT Calculations of X-ray Spectroscopic Data: Calculations of the Pu M 5 -edge RIXS and VB-RIXS maps of PuO 2 were adapted from the ligand-field densityfunctional theory (LFDFT) method 111 as implemented in the Amsterdam density functional (ADF) program version 2023. LFDFT has been shown to be well-suited for actinide complexes. The RIXS and VB-RIXS maps were modeled by using the polarizability tensor for inelastic scattering via the Kramers-Heisenberg formula. The ground state (GS), intermediate state (IS), final state (FS), and valence-band final state (VB-FS) electron configurations used for Pu were 5f 4 , 3d 9 5f 5 , 4f 13 5f 5 , and 5f 5 L, respectively, with L indicating that in VB-FS a hole was created in the occupied valence orbitals (cf. Figure ). The LFDFT calculations were performed in two steps. First, based on the PuO 8 12-embedded cluster model, single-point DFT calculations were performed with the PBE0 functional and all-electron Slater-type orbital (STO) basis sets TZ2P (for Pu) and TZP (for O), using an 'average of configurations' (AOC) 116 approach with fractional electron occupations of selected molecular orbitals. Note that the ADF code does not support the use of ECPs, and therefore 230 nearest-ion point charges were used for the embedding (78 Pu 4+ and 152 O 2-). For the GS, for example, 4 electrons were evenly distributed over the seven molecular orbitals of predominant Pu 5f character. The GS, IS, FS and VB-FS electronic structures are characterized in the SI, Figure , with respect to the projected DOS for Pu and O atoms. In a second step, the ligand-field potential was extracted from the calculations of the first step to generate the Slater-Condon integrals, spin-orbit coupling constants, the charge-transfer hopping energy, and orbital hybridization, without empirical corrections or scaling factors. The calculated parameters together with reference data for comparison are listed in the SI, Tables S8-S10. These parameters were used to set up the effective and charge transfer Hamiltonian 117 to calculate the multiplet energies and ligand-field wavefunctions for the various electron configurations, and to model the Pu M 5 -edge RIXS and VB-RIXS spectra. ] > 99%), and a slightly higher concentration [Pu 3+ ] = 109 mM (cf. Figure ). The latter is likely a result of partial evaporation of solvent over the course of the experiments. Vis/NIR absorption spectra of liquid Pu samples were recorded on a diode array spectrometer (MCS 501 system equipped with a halogen lamp source system CLH 500 from Zeiss, single beam mode, wavelength range 350-1020 nm, integration time 12 ms, data interval ≈ 0.8 nm, PMMA cuvettes with 10 mm path length from Brand). All spectra were recorded inside of argon glove boxes by connecting the inside cuvette holder to the outside spectrometer via fiber optics. The aqueous Pu 4+ (aq) sample was prepared by electrochemical oxidation of a 242 Pu 3+ stock solution (5.3 mmol/L in 2 M HClO 4 ) and characterized by Vis/NIR spectroscopy after the HR-XANES experiments (Figure ). 210 μL of each solution was filled into a PEEK sample cell with a 12.5 μm KAPTON® foil. The surface contamination-free cells were enclosed by a second containment. Details on the electrochemical oxidation and the sample cells can be found in Reference 18.