On the performance of second-order approximate coupled-cluster singles and doubles methods for non-valence anions

We investigate the capability of several variants of the second-order approximate coupled-cluster singles and doubles (CC2) method to describe dipole-bound, quadrupole-bound, and correlation-bound molecular anions. The binding energy of anions formed by electron attachment to closed-shell molecules is computed using the electron attachment variant of CC2 (EA-CC2), whereas anions with a closed-shell ground state are treated with the standard CC2 method that preserves the number of particles. We find that EA-CC2 captures the binding energies of dipole-bound radical anions quite well, whereas results for other types of non-valence anions are less reliable. We also test the performance of semi-empirical spin-scaling factors for all types of non-valence anions and observe that the spin-scaled CC2 variants generally do not provide more accurate binding energies for dipole-bound anions, while the binding energies of quadrupole-bound and correlation-bound anions are improved. As exemplary applications of EA-CC2, we investigate the dipole-bound anions of the steroids cortisol, progesterone, and testosterone. In addition, we characterize electron attachment to sym-tetracyanonaphthalene, a molecule that supports five anionic states, two of which can be interpreted as hitherto unobserved π-type quadrupole-bound states.