What is the lowest-energy isomer of the C-60 dimer?

The C-60 dimer is studied using fully relaxed Hartree-Fock, local density, and gradient-corrected density functional calculations both for the neutral and doubly charged case (C-120(2-)). In the neutral case, the [2 + 2] cycloadduct is lower in energy than the singly bonded (SE) isomer formed by direct covalent bonding between two C-60 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 C-60 molecule, however, our best calculations indicate that the relative energy ordering between the [2 + 2] and SE structures reverts. Thus, the SE dimer is predicted to be more stable than the [2 + 2] isomer in mono-anionic fullerides.