Multi-reference Approach to the Computation of Double-Core-Hole Spectra

Double-Core Hole (DCH) states of small molecules are assessed with the restrictedactive space self-consistent field (RASSCF) and multi-state restricted active space perturbationtheory of second order (MS-RASPT2) approximations. To ensure an unbiaseddescription of the relaxation and correlation effects on the DCH states, the neutralground state and DCH wave functions are optimized separately, whereas the spectralintensities are computed with a biorthonormalized set of molecular orbitals within thestate-interaction (SI) approximation. Accurate shake-up satellites binding energies andintensities of double-core-ionized states (K-2) are obtained for H2O, N2, CO and C2H2n(n=1–3). The results are analyzed in details and show excellent agreement with recentexperimental data.