The experimental VB-RIXS spectrum for Pu exhibits a similar profile as for UO 2 in Reference 73. It is characterized by an intense peak (Figure ) that describes the photons elastically scattered by the sample (at 0 eV, not modeled in LFDFT) and a weaker peak at lower emission energy representing the valence-band signals. The energy of this elastic peak and the maximum of the primary absorption peak of the experimental Pu M 5 -edge HR-XANES spectrum coincide. The calculated VB-RIXS spectra (Figure ) agree well with the experiment, demonstrating that there is mixing of Pu 5f with O 2p electron density, which is evidence for participation of the Pu 5f shell in the Pu-O covalent bonding. Note that the LFDFT results show that the 5f occupation in the valence band decreases in the VB-FS (see the Pu 5f projected DOS as function of the GS, IS, FS and VB-FS in Figure ). In the VB-FS electronic structure a hole is present in the occupied valence band (valence exciton), ultimately pushing the partially filled Pu 5f states to higher energy.