The emergence of high-resolution and time-resolved X-ray spectroscopy demands accurate and efficient electronic structure theories for core-excited states. To compute these core-excited states, we extended our multireference driven similarity renormalization (MR-DSRG) method by combining it with a GASSCF treatment of orbital relaxation and static electron correlation effects. We have applied different levels of MR-DSRG treatments including perturbative theories (DSRG-MRPT2/3) and iterative theory [MR-LDSRG(2)] in our study. These methods are benchmarked on the C, N, and O K-Edge transitions of more than 40 gas-phase molecules and the vibrational structure of these transitions is studied in-depth for five diatomic molecules. The GASSCF-MR-DSRG theories show good agreement with the energies and intensities of core-excited peaks and vibrational structure from experimental X-ray absorption spectroscopy<b>. </b>We find that a treatment of dynamical correlation that goes beyond the second-order level in perturbation theory improves the accuracy of electronic structure calculations, especially in the bond-dissociation region.

Theoretical computation of X-ray absorption spectroscopy with multi-reference driven similarity renormalization group theory

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