Describing Inner-shell Ionization using Multiconfigurational Wave Functions

The Multiconfigurational Self-Consistent Field (MCSCF) method is a powerful approach to describe atomic and molecular systems. In previous papers, we show that surprisingly good quantitative results are presented to core excited states when using the IS-MCSCF procedure. A broad evaluation of the IS-MCSCF method and its variations can provide useful knowledge about its capabilities and limitations. We applied it extensively to investigate the K-shell ionization of a selected group of interesting small molecules (CO, O₂, and N₂). We evaluate principally different constructions of the MCSCF wave function and different methods to recover post-MCSCF electronic correlation, among other theoretical details.

We have investigated the performance of several forms of IS-MCSCF wave functions, like IS-GVB-PP, IS-FVBL, and IS-CASSCF. Transition energy and potential energy curves show that all forms of the IS-MCSCF function lead to a good description of the inner-shell state. Multireference Configuration Interaction (MRCI) did not succeed in improving the results derived from IS-MCSCF approaches, despite the high computational demand. In contrast, the GMC-QDPT method presents nice low-cost improvements to the ionized core states’ description, as observed previously for excited states.

We found that even the simplest form of the theory, IS-GVB-PP or IS-GVB-PP/GMC-QDPT can lead to a quantitative agreement with experimental core-shell ionization potential. This has not been verified in valence transitions in essentially any form of multiconfigurational function, not even CASSCF.

Moura, C., Oliveira, R., Rocha, A. (2013). Transition energy and potential energy curves for ionized inner-shell states of CO, O₂ and N₂ calculated by several inner-shell multiconfigurational approaches, Journal of Molecular Modeling 19(5), 2027-2033. https://dx.doi.org/10.1007/s00894-012-1622-x