A Fresh Look at Signatures of <i>s</i> -Wave Scattering: Symmetry and the Breakdown of the Born–Oppenheimer Approximation

resonance. Strong interaction leads to the breakdown of the Born-Oppenheimer approximation, which results in a finite probability to decay into the detachment continuum even in regions where the anionic state is energetically below the neutral state. We show that calculations with a complex absorbing potential capture this special character of σ-type anions through removal of the detachment continuum, affording diabatization of the anionic state while describing the decay into the continuum via the imaginary energy.