Computing X-ray absorption spectra from linear-response particles atop optimized holes

Abstract: State specific orbital optimized density functional theory (OO-DFT) methods like restricted open-shell Kohn-Sham (ROKS) can attain semiquantitative accuracy for predicting X-ray absorption spectra of closed-shell molecules. OO-DFT methods however require that each state be individually optimized. In this work, we present an approach to generate an approximate core-excited state density for use with the ROKS energy ansatz, that is capable of giving reasonable accuracy without requiring state-specific optimization. This is achieved by fully optimizing the core-hole through the core-ionized state, followed by use of electron-addition configuration interaction singles (EA-CIS) to obtain the particle level. This hybrid approach can be viewed as a DFT generalization of the static-exchange (STEX) method, and can attain $\sim 0.6$ eV RMS error for the K-edges of C-F through the use of local functionals like PBE and OLYP. This ROKS(STEX) approach can also be used to identify important transitions for full OO ROKS treatment, and can thus help reduce the computational cost for obtaining OO-DFT quality spectra. ROKS(STEX) therefore appears to be a useful technique for efficient prediction of X-ray absorption spectra.