Bound electronic states of the smallest fullerene C-20(-) anion

We report on high-level coupled-cluster calculations for the anion states of the smallest fullerene C-20. Using the state-of-the-art EA-EOM-CCSD method we revealed that the C-20(-) anion has five bound electronic states at the C-20 neutral ground-state D-3d equilibrium configuration. These are two pairs of E-2(u) and (2)A(2u) states and one (2)A(1g) state. The binding energies vary from 2.05 eV for the most bound E-2(u) state to <1 meV for the (2)A(1g) state. An analysis in terms of radial and angular density distribution of the excess electron revealed that the two E-2(u)/(2)A(2u) pairs are valence-like states while (2)A(1g) corresponds to a super-atomic-like (SAMO) state. The valence states of the first E-2(u)/(2)A(2u) pair were found to be of p-type whereas those of the second pair are of hybrid sp-type. We have also applied a simple model to understand the binding of the excess electron in C-20(-). The model confirms the existence of only one SAMO state of the s-type for C-20(-). At the same time, it overestimates the number of the bound valence states of C-20(-).