Core Excitations of Uranyl in Cs₂UO₂Cl₄ from Relativistic Embedded Damped Response Time-Dependent Density Functional Theory Calculations

Abstract

X-ray spectroscopies, by their high selectivity and sensitivity to the chemical environment around the atoms probed, provide significant insights into the electronic structures of molecules and materials. Interpreting experimental results requires reliable theoretical models, accounting for environmental, relativistic, electron correlation, and orbital relaxation effects in a balanced manner. In this work, we present a protocol for the simulation of core excited spectra with damped response timedependent density functional theory based on the Dirac−Coulomb Hamiltonian (4c-DR-TD-DFT), in which environmental effects are accounted for through the frozen density embedding (FDE) method. We showcase this approach for the uranium M 4 -and L 3edges and oxygen K-edge of the uranyl tetrachloride (UO 2 Cl 4 2− ) unit as found in a host Cs 2 UO 2 Cl 4 crystal. We have found that the 4c-DR-TD-DFT simulations yield excitation spectra that very closely match the experiment for the uranium M 4 -edge and the oxygen K-edge, with good agreement for the broad experimental spectra for the L 3 -edge. By decomposing the complex polarizability in terms of its components, we have been able to correlate our results with angle-resolved spectra. We have observed that for all edges, but in particular the uranium M 4 -edge, an embedded model in which the chloride ligands are replaced by an embedding potential reproduces rather well the spectral profile obtained for UO 2 Cl 4 2− . Our results underscore the importance of the equatorial ligands to simulating core spectra at both uranium and oxygen edges.

Quantifying Covalency and Environmental Effects in RASSCF-Simulated O K-Edge XANES of Uranyl

“…In contrast, TDDFT simulations, which do not capture such relaxations, report peaks that are approximately 20 eV higher than experimental positions . Published 4c-DR-TDDFT calculations by Misael et al show substantial improvement on peak predictions over the TDDFT reported in Table . These simulations included equatorial chloride ligands and the resulting discrepancy in peak positions is reduced to between 11–14 eV.…”

“…Gomes et al, later showed that capturing the Cs 2 UO 2 Cl 4 crystal environment more completely through incorporation of the Cs counterions had only a small improvement on predicting the relative position of excited states. A recent 4c-DR-TDDFT simulation of uranyl O K-edge XANES by Misael et al, showed that relative peak positions can be brought into closer agreement with experiment by the inclusion of equatorial chloride ligands. However, this study also showed that the modeling of the counterions as an embedding potential did not significantly alter predictions.…”

Unveiling Hidden Shake-Up Features in the Uranyl M4-Edge Spectrum

“…Previous works have shown that modeling X-ray spectra using time-dependent density functional theory (TDDFT) with relativistic corrections is capable of capturing some of the spectral features in uranyl core-excitations and yields qualitatively correct spectra. − However, the correlation captured by TDDFT is incomplete, shown by the importance of double excitations in the accurate modeling of M-edge spectra in multireference methods with perturbative spin–orbit coupling. ,, On the other hand, the manifestation of double excitation and its physical characteristics in the uranyl M-edge spectrum remain largely unexplored.…”