Core-Excited States and X-ray Absorption Spectra from Multireference Algebraic Diagrammatic Construction Theory

Wereport the development and benchmark of multireference algebraicdiagrammatic construction theory (MR-ADC) for the simulations of core-excitedstates and X-ray absorption spectra (XAS). Our work features an implementationthat incorporates core-valence separation into the strict and extendedsecond-order MR-ADC approximations (MR-ADC(2) and MR-ADC(2)-X), providingefficient access to high-energy excited states without including inner-shellorbitals in the active space. Benchmark results on a set of smallmolecules indicate that at equilibrium geometries, the accuracy ofMR-ADC is similar to that of single-reference ADC theory when staticcorrelation effects are not important. In this case, MR-ADC(2)-X performssimilarly to single- and multireference coupled cluster methods inreproducing the experimental XAS peak spacings. We demonstrate thepotential of MR-ADC for chemical systems with multiconfigurationalelectronic structure by calculating the K-edge XAS spectrum of theozone molecule with a multireference character in its ground electronicstate and the dissociation curve of core-excited molecular nitrogen.For ozone, the MR-ADC results agree well with the data from experimentaland previous multireference studies of ozone XAS, in contrast to theresults of single-reference methods, which underestimate relativepeak energies and intensities. The MR-ADC methods also predict thecorrect shape of the core-excited nitrogen potential energy curve,and are in good agreement with accurate calculations using drivensimilarity renormalization group approaches. These findings suggestthat MR-ADC(2) and MR-ADC(2)-X are promising methods for the XAS simulationsof multireference systems and pave the way for their efficient computerimplementation and applications.