Electronic excitations of C₆₀ fullerene calculated using the ab initio cluster expansion method

The electronic excited states and optical absorption spectrum of C₆₀ fullerene below 6.2 eV (200 nm) were calculated using the ab initio many-body wavefunction theory of cluster expansion method: the symmetry-adapted cluster-configuration interaction method. Not only optically allowed states but also optically forbidden states were calculated for studying the observed weak absorptions in the visible region. The lowest calculated singlet excited state was the 1¹G_g state. The electron correlation effects are important in determining the energy levels of the four low-lying states that have the character of degenerated HOMO–LUMO transition. The lowest optically allowed 1¹T_1u state was calculated at 3.67 eV; this is significantly higher than the energy values found in previous density functional calculations. The observed weak absorption around 3.08 eV appears to correspond to the optically forbidden 1¹T_2u state with intensity borrowing via vibronic couplings.