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 and multi-state perturbation theory second order approximations. To ensure an unbiased description relaxation correlation effects on DCH states, neutral ground-state wave functions optimized separately, whereas spectral intensities computed a biorthonormalized set molecular orbitals within state-interaction approximation. Accurate shake-up satellite binding energies double-core-ionized (K?2) obtained for H2O, N2, CO, C2H2n (n = 1–3). The results analyzed in detail show excellent agreement recent theoretical experimental data. K?2 spectra H2O here completely characterized first time.