title: On the nature of electron correlation in C60.
abstract: The ground state restricted Hartree Fock (RHF) wave function of C(60) is found to be unstable with respect to spin symmetry breaking, and further minimization leads to a significantly spin contaminated unrestricted Hartree Fock (UHF) solution (<S(2)> = 7.5, 9.6 for singlet and triplet, respectively). The nature of the symmetry breaking in C(60) relative to the radicaloid fullerene, C(36), is assessed by energy lowering of the UHF solution, <S(2)>, and the unpaired electron number. We conclude that the high value of each of these measures in C(60) is not attributable to strong correlation behavior as is the case for C(36). Instead, their origin is from the collective effect of relatively weak, global correlations present in the π space of both fullerenes. Second order perturbation (MP2) calculations of the singlet triplet gap are significantly more accurate with RHF orbitals than UHF orbitals, while orbital optimized opposite spin second order correlation (O2) performs even better.
authors: Department of Chemistry, University of California, Berkeley, California 94720, USA.
authors: David
authors: D
authors: Stück
authors: Thomas A
authors: TA
authors: Baker
authors: Paul
authors: P
authors: Zimmerman
authors: Westin
authors: W
authors: Kurlancheek
authors: Martin
authors: M
authors: Head-Gordon