Fundamental excitations inC60

The geometrical and electronic structure of the neutral ${\mathrm{C}}_{60}$ molecule and of ${\mathrm{C}}_{60}^{\mathit{X}\mathrm{-}}$ anions (X=1, 2, and 3) is calculated using a Pariser-Parr-Pople-type of Hamiltonian in combination with single and double configuration interaction. The model couples the bond length to the bond order, as well as to charge-charge and bond-order–bond-order interactions between nearest-neighbor sites. The total energy is minimized with respect to variations in the bond lengths. Although the energy differences between the molecular orbitals close to the Fermi energy are small, correlation effects in terms of configuration interaction are observed to be of minor importance for the ground-state geometrical and electronic structure. In accordance with this result, the cyclic polaronic defect (Jahn-Teller distortion of the molecule along the equatorial line) produced by Hückel-type calculations for ${\mathrm{C}}_{60}^{\mathit{X}\mathrm{-}}$ anions is observed to be stable within the correlated description presented in this study. Exactly the same type of defect also exists in the relaxed structure of the first excited state of the neutral molecule. Thus, the cyclic polaron corresponds to a low-lying fundamental excitation in ${\mathrm{C}}_{60}$.