AB-INITIO MOLECULAR-ORBITAL CALCULATION FOR C70 AND 7 ISOMERS OF C80

We investigate stable structures and electronic properties for isolated C70 and nonisolated C80, by performing an ab initio molecular-orbital calculation based on a nonlocal density-functional formalism and a Harris-functional approximation. We first show the stable structure for C70 and give the explanation for the experimental inconsistency in structure by revealing the existence of a metastable structure. Concerning the seven isomers of C80 satisfying an isolated pentagon rule, it is found that the isomer with nearly D2 symmetry (D2-C80) has a large bond energy and highest-occupied molecular-orbital-lowest unoccupied molecular-orbital (HOMO-LUMO) energy gap which are comparable to those for isolated higher fullerenes. Therefore we propose that D2-C80 should be detectable in an experiment. Since C80 does not belong to the sequence of fullerenes with magic numbers, we discuss the problem of whether it is really unstable or not from the viewpoint of the electronic properties. Further, we consider the structures of C80 ions and show the great stability of the C-80(6-) with nearly I(h) symmetry which has a larger HOMO-LUMO gap than both C60 and C70.