Positronium negative ion: Molecule or atom?

A highly accurate calculation is supplemented by an adiabatic approximation to explore the resonance spectrum of positronium ${\mathrm{Ps}}^{\mathrm{-}}$. Surprisingly, the spectrum can be understood and classified with ${\mathrm{H}}_2^{+}$ quantum numbers by treating the interelectronic axis of ${\mathrm{Ps}}^{\mathrm{-}}$ as an adiabatic parameter. We report and interpret the existence of ${}_{}{}^1{}_{}{}^{}$S shape resonances, a phenomenon so far unknown in three-body Coulomb systems. The new results on ${\mathrm{Ps}}^{\mathrm{-}}$ combined with previous results for ${\mathrm{H}}^{\mathrm{-}}$ suggest the existence of a resonance spectrum and its similarity for all ABA Coulomb systems with charges ‖${\mathit{Z}}_{\mathit{A}}$/${\mathit{Z}}_{\mathit{B}}$‖=1 and masses ${\mathit{m}}_{\mathit{A}}$/${\mathit{m}}_{\mathit{B}}$≥1.