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

Double-Core Hole (DCH) states of small molecules are assessed with the restricted active space self-consistent field (RASSCF) and multi-state restricted active space perturbation theory of second order (MS-RASPT2) approximations. To ensure an unbiased description of the relaxation and correlation effects on the DCH states, the neutral ground state and DCH wave functions are optimized separately, whereas the spectral intensities are computed with a biorthonormalized set of molecular orbitals within the state-interaction (SI) approximation. Accurate shake-up satellites binding energies and intensities of double-core-ionized states (K-2) are obtained for H2O, N2, CO and C2H2n (n=1–3). The results are analyzed in detail and show excellent agreement with recent theoretical and experimental data. The K^{-2} shake-up spectra of H2O and the C2H2n molecules are here completely characterized for the first time.