ELSEVIER Journal of Molecular Structure (Theochem) 311 (1994) THEO CHEM Electronic excitations in fullerenes: Jahn- Teller distorted structures of C 60 Peter R. Surjan'v":", Laszlo Udvardi", Karoly Nemeth" "Department of Theoretical Chemistry, Eotvos University, H-1518 P.O. Box 32, Budapest 112, Hungary "Quanium Theory Group, Institute of Physics, Technical University, Budafoki ut 8, H-1521 Budapest, Hungary (Received 19 July 1993; accepted 14 September 1993) Abstract The ground state wavefunction of the neutral icosahedral C 60 molecule belongs to the total symmetric one-dimen- sional A g representation. However, the degeneracy of the HOMO is five-fold while the LUMO is triply degenerate. This means that the lowest excited and ionized many-electron states will also be degenerate, and thus they are subjected to Jahn- Teller distortions. In this work we used a simple model Hamiltonian to study the extent and energy of the distortions arising due to excitations. For the -r-electrons we used a Pariser-Parr-Pople type wavefunction augmented by an empirical potential to describe the (J cores. The zr-electron part of the Hamiltonian depends on the bond lengths which can be optimized by means of simple gradient techniques. The excited states are described by the Tamm - Dancoff approximation (all single CI). In order to locate the distorted states on the energy hypersurface, the degenerate excited states were reduced according to those subgroups of the I h group that contain one-dimensional (that is, Jahn--Teller inactive) irreducible representations in an excitation subspace. Distorted structures of D 2h , D 3d , D Sd and T h symmetries were determined. The extent of the distortions is small, the largest change in bond lengths being 0.02 A. The Jahn Teller distortion energies were found to be typically I kcal mol ": Singlet and triplet excited state spectra were computed using the CNDO/S-CI method. Comparison of the calculated spectra with experimental results shows evidence of Teller distortions. 1. Introduction In spite of the thousands of papers that are presently being published on fullerenes, these highly symmetric novel materials still continue to attract investigations in basic research (1-4]. A detailed theoretical description of the electronic and geometrical structure of individual ground state C 60 molecules has already been elucidated at various levels, including semiempirical and *Corresponding author. more sophisticated (ab initio) quantum chemical methods [5-9], including the approximate treatment of electron correlation [10, 11]. An important feature of C 60 is that it belongs to the icosahedral point group lh' which is the largest finite subgroup of the rotation group 03; thus this molecule exhibits the highest possible finite symmetry in nature. The ground state wavefunction is of symmetry A 1g (the dominant configuration comes from completely filled levels) which is one-dimensional. The lh group possesses three-, four- and five-dimensional representations 0166-1280/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0166-1280(93)03516-A