What is the lowest-energy isomer of the C60 dimer?

The C60 dimer is studied using fully relaxed Hartree-Fock, local density, and gradient-corrected density functional calculations both for the neutral and doubly charged case (C1202−). In the neutral case, the [2 + 2] cycloadduct is lower in energy than the singly bonded (SB) isomer formed by direct covalent bonding between two C60 molecules, an isomer for which no minimum in the potential energy surface was found at high levels of theory. With the addition of one electron to each C60 molecule, however, our best calculations indicate that the relative energy ordering between the [2 + 2] and SB structures reverts. Thus, the SB dimer is predicted to be more stable than the [2 + 2] isomer in mono-anionic fullerides.