  We compute EOM-EA-CCSD and EOM-EA-CCSDT potential energy curves and
one-electron properties of several anions at bond lengths close to where these
states become unbound. In the potential energy curves of the totally symmetric
anions of HCl and pyrrole, which are associated with s-wave scattering states
at the equilibrium bond lengths of the parent neutral molecules, we observe on
inclusion of diffuse basis functions a pronounced bending effect near the
crossing points with the potential energy curves of the neutral molecules.
Additionally, we observe that the Dyson orbital and second moment of the
electron density become extremely large in this region. In particular, the
second moment of the HCl anion becomes 5 orders of magnitude times larger over
a range of 5 pm. This behaviour is very different to the well-characterised
non-totally symmetric anions of dinitrogen and dihydrogen that correspond to
electronic resonances at the equilibrium bond lengths of their parent neutral
molecules. Our work thus shows that bound state electronic-structure methods
can distinguish between anions that turn into electronic resonances and those
associated with s-wave scattering states.
