Satpathy S., Antropov V.P., Andersen O.K., Jepsen O., Gunnarsson O., Liechtenstein A.I.
The authors discuss the geometries of crystalline FCC C60 for three cases of directional order, the hypothetical unidirectional structure with space group Fm3, the bidirectional structure (P42/mnm) which is the ordered version of the structure observed for alkali-metal-doped compounds at room temperature, and the quadridirectional, low-temperature structure (Pa3) of pure C60. Analytical, parameter-free expressions for the t1u wave functions are derived and used to obtain analytical conduction-band Hamiltonians for all three structures. The interactions with other subbands are included in numerical tight-binding calculations with a basis of 60 radial carbon orbitals per molecule. Ab initio density-functional (local-density approximation (LDA)) calculations are performed for unidirectional FCC C60 and RbC60 for different lattice constants. The authors use the linear-muffin-tin-orbitals (LMTO) method in the atomic-spheres approximation with carefully chosen interstitial spheres. The LDA bands are compared with photoemission and inverse photoemission data for C60. For RbC60 they find that the alkali-metal atom is fully ionized and that the doped electron occupies the t1u band in a rigid-band-like fashion. Tight-binding theory explains why, and indicates that this holds generally for An-xBxC60 with n
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