Structure and energetics of neutral and negatively charged C60 dimers

Ground state optimized geometries and total energies have been determined for the neutral and the doubly negative charged C60 dimers of various structures. Semi-empirical calculations have been carried out using MNDO, AM1 and PM3 parametrization. For the neutral and the charged dimers different structures have been found at the global energy minima. In the neutral case, the well known 66/66 cycloaddition product with D2h symmetry was found to be the most stable configuration. In contrast, the charged dimer had a different structure: the AM1 and PM3 calculations showed that it is bound through a single carbon atom on each C60 moiety, the two balls being in trans position (C2h symmetry). This is in accord with X-ray results on the quenched phase of A1C60 (A = K, Rb).