Molecular resonances by removing complex absorbing potentials via Padé; Application to CO^- and N₂

Atomic and molecular resonances play a role in many physical processes. Resonances are metastable states that have a finite lifetime. Nowadays, resonance complex eigenvalues can be calculated by quantum chemistry packages modified to include complex absorbing potentials (CAPs), where the imaginary energy part corresponds to the resonance lifetime. CAPs provide similar conditions to the ones obtained by imposing outgoing boundary conditions (OBCs), however, they also introduce artificial reflections. In this study, we present a scheme that eliminates the artificial effect of CAPs from bona fide electronic-structure calculations using the Padé approximant. This allows us to reach the physical limit and obtain the same eigenvalues as if we were able to solve the time-independent Schrödinger equation with OBCs (and without adding an unphysical CAP). We apply this scheme to the CO^- and N₂^- shape-type ²Π resonances with excellent agreement with experimental and other theoretical studies. The strength of the presented approach is demonstrated by providing accurate resonance complex energies independent of the CAP location, even when placed on the nuclei, which makes the optimization of the CAP-onset parameters redundant.