DFT + U Simulation of the X-ray Absorption Near-Edge Structure of Bulk UO₂ and PuO₂

Hubbard U-corrected density functional theory within the periodic boundary condition model in the WIEN2k code is used to simulate the actinide L_III and O K edge X-ray absorption near-edge structure (XANES) for UO₂ and PuO₂. Spin-orbit coupling effects are included, as are possible excitonic effects using supercells with a core hole on one of the atoms. Our calculations yield spectra in excellent agreement with previous experiments and superior to previous simulations. Density of states analysis reveals the mechanism behind the XANES peaks: the main contribution to the U/Pu L_III edges comes from the U/Pu d states hybridized with O p states, while as expected, the O p states primarily determine the O K edges of both UO₂ and PuO₂. The O K edges also feature O p hybridizing with U/Pu d and f states in the low-energy region and with U/Pu s and p states for the higher-energy peaks.