On the nature of electron correlation in C60

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 (<< S2 >> = 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, << S2 >>, 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 pi 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. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3661158]