Core-Excited States for Open-Shell Systems in Similarity-Transformed Equation-of-Motion Theory.

X-ray absorption spectroscopy (XAS) is a powerful method for exploring molecular electronic structure by exciting core electrons into higher unoccupied molecular orbitals. In this study, we present the first integration of the spin-unrestricted similarity-transformed equation-of-motion coupled cluster method (CVS-USTEOM-CCSD) for core-excited and core-ionized states into the ORCA quantum chemistry package. Using the core-valence separation (CVS) approach, we evaluate the accuracy of CVS-USTEOM-CCSD across 13 open-shell organic systems, covering over 20 core excitations with diverse spin multiplicities (doublet, triplet, and quartet). The implementation leverages automated active space selection, incorporating CIS natural orbitals to efficiently capture electronic transitions. We benchmark the predicted K- and L-edge spectra against experimental data, underscoring the accuracy of the CVS-USTEOM-CCSD method for high-precision core excitation studies.