Quantum chemical calculations on the buckminsterfullerene (C60) system

Abstract

Abstract We performed ab initio SCF LCAO calculations for a single C60 molecule and for interacting undoped C60 systems with different lattice constants (corresponding to the undoped case and to the K3C60 and Rb3C60 crystals, respectively). We also used different orbitals for different spin computations on single K3C60 and Rb3C60 molecules. To obtain convergence of the large number of basis functions in both programs (which belong to the Gaussian 90 package), we applied the direct SCF option. The analysis of the σ- and π-type MO splittings showed that in all cases the σ splittings are at least one order of magnitude smaller than the π splittings in the region of the Fermi level. This raises the possibility of explaining the likely electronic contribution to the superconductivity of doped C60 systems by using the so-called “two band” model. In this model the polarization of the localized electrons in filled narrow bands gives rise to an effective attraction between the delocalized electrons in the broad metallic band which facilitates Cooper pair formation. A simple mathematical derivation has shown that within the framework of the ‘two-band’ model, the decrease in level splittings with the increase in lattice constants (a0) (which we calculated for two C60 molecules) approximately accounts for the linear increase in the super-conducting transition temperature (Tc) with a0 or its linear decrease with pressure. These results are in quite good agreement with the experimental findings.