Ground and excited electronic structures of metal encapsulated nanocages: the cases of endohedral M@C20H20 (M = K, Rb, Ca, Sr) and M@C36H36 (M = Na, K, Rb).

High-level electronic structure calculations were performed to analyze ground and excited states of neutral and cationic endohedral M@C20H20 (M = K, Rb, Ca, Sr) and M@C36H36 (M = Na, K, Rb). In their ground states, one or two electrons occupy a diffuse atomic s-type orbital, thus 1s1 and 1s2 superatomic electronic configurations are assigned for M = Na, K, Rb and M = Ca, Sr cases, respectively. These species populate 1p-, 1d-, 1f-superatomic orbitals in electronically excited states. The specific superatomic Aufbau model introduced for M@C20H20 (M = K, Rb) is 1s, 1p, 1d, 2s, 1f, 2p, 2d, 1g, 2f. On the other hand, excited electronic spectra of M@C20H20 (M = Ca, Sr) are rich in multireference characters. Excited states of bigger M@C36H36 molecules were investigated up to the 1d level and the transitions were found to require slightly higher energies compared to M@C20H20. These superatoms possess lower ionization potentials, hence can also be categorized as superalkalis.