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State-specific orbital optimized approaches are more accurate at predictingcore-level spectra than traditional linear-response protocols, but theirutility had been restricted on account of the risk of `variational collapse'down to the ground state. We employ the recently developed square gradientminimization (SGM, J. Chem. Theory Comput. 16, 1699-1710, 2020) algorithm toreliably avoid variational collapse and study the effectiveness of orbitaloptimized density functional theory (DFT) at predicting second period element1s core-level spectra of open-shell systems. Several density functionals(including SCAN, B3LYP and $\omega$B97X-D3) are found to predict excitationenergies from the core to singly occupied levels to high accuracy ($\le 0.3$ eVRMS error), against available experimental data. Higher excited states arehowever more challenging by virtue of being intrinsically multiconfigurational.We thus present a CI inspired route to self-consistently recouple singledeterminant mixed configurations obtained from DFT, in order to obtainapproximate doublet states. This recoupling scheme is used to predict the CK-edge spectra of the allyl radical, the O K-edge spectra of CO$^+$ and the NK-edge of NO$_2$ to high accuracy relative to experiment, indicatingsubstantial promise in using this approach for computation of core-levelspectra for doublet species (vs more traditional time dependent DFT, EOM-CCSDor using unrecoupled mixed configurations). We also present general guidelinesfor computing core-excited states from orbital optimized DFT.

journal of chemical physics online/the journal of chemical physics/journal of chemical physics

Accurate prediction of core-level spectra of radicals at density  functional theory cost via square gradient minimization and recoupling of  mixed configurations