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We report 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), providing anefficient access to high-energy excited states without including inner-shellorbitals in the active space. Benchmark results on a set of small moleculesindicate that at equilibrium geometries the accuracy of MR-ADC is similar tothat of single-reference ADC theory when static correlation effects are notimportant. In this case, MR-ADC(2)-X performs similarly to single- andmultireference coupled cluster methods in reproducing the experimental XAS peakspacings. We demonstrate the potential of MR-ADC for chemical systems withmulticonfigurational electronic structure by calculating the K-edge XASspectrum of the ozone molecule with a multireference character in its groundelectronic state and the dissociation curve of core-excited molecular nitrogen.For ozone, the MR-ADC results agree well with the data from experimental andprevious multireference studies of ozone XAS, in contrast to the results ofsingle-reference methods, which underestimate relative peak energies andintensities. The MR-ADC methods also predict the correct shape of core-excitednitrogen potential energy curve, in a good agreement with accurate calculationsusing driven similarity renormalization group approaches. These findingssuggest that MR-ADC(2) and MR-ADC(2)-X are promising methods for the XASsimulations of multireference systems and pave the way for their efficientcomputer implementation and applications.

Core-Excited States and X-Ray Absorption Spectra From Multireference Algebraic Diagrammatic Construction Theory