Multi-Reference Approach to Normal and Resonant Auger Spectra Based on the One-Center Approximation

A methodology to calculate the decay rates of normal and resonant Auger processes in atoms and molecules based on the One-Center Approximation (OCA) using atomic radial Auger integrals is implemented within the restricted-active-space self-consistent-field (RASSCF) and multi-state restricted-active-space perturbation theory of second order (MS-RASPT2) frameworks as part of the OpenMolcas project. To ensure an unbiased description of the correlation and relaxation effects on the initial core excited/ ionized states and the final cationic states, their wave functions are optimized independently, whereas the Auger matrix elements are computed with a biorthonormalized set of molecular orbitals within the state-interaction (SI) approach. As a decay of an isolated resonance, the computation of Auger intensities involves matrix elements with one electron in the continuum. However, treating ionization and autoionization problems can be overwhelmingly complicated for non-experts because of many peculiarities in comparison to bound-state electronic structure theory. One of the advantages of our approach is that, by projecting the intensities on the atomic center bearing the core hole and using precalculated atomic radial two-electron integrals, the Auger decay rates can be easily obtained directly with OpenMolcas, avoiding the need to interface it with external programs to compute matrix elements with the photoelectron wave function. The implementation is tested on the Ne atom, for which numerous theoretical and experimental data are available for comparison, as well as on a set of prototype closed- and open-shell molecules, namely, CO, N2, HNCO, H2O, NO2, and C4N2H4 (pyrimidine).