Theoretical study of molecular electronic excitations and optical transitions of C₆₀

We report results of ab initio calculations of excited states of the fullerene molecule using the configuration-interaction approach with singly excited determinants (SCI). We have used both experimental geometry and the geometry optimized by the density functional method and worked with basis sets at the correlation-consistent polarization valence triple-ζ (cc-pVTZ) and augmented cc-pVTZ levels. In contrast to the early SCI semiempirical calculations, we find that the two lowest ¹T_1u←¹A_g electron optical lines are situated at relatively high energies of ∼5.8 eV (214 nm) and ∼6.3 eV (197 nm). These two lines originate from two ¹T_1u←¹A_g transitions: from the highest occupied molecular orbital (HOMO) to one above the lowest unoccupied molecular orbital (LUMO+1) (6h_u→3t_1g) and from (HOMO−1) to LUMO (10h_g→7t_1u). The lowest molecular excitation, which is the 1 ³T_2g level, is found at ∼2.5 eV. Inclusion of doubly excited determinants leads only to minor corrections to this picture. We discuss possible assignment of absorption bands at energies smaller than 5.8 eV (or λ larger than 214 nm).