Ab initiomolecular-orbital calculation forC70and seven isomers ofC80

We investigate stable structures and electronic properties for isolated ${\mathrm{C}}_{70}$ and nonisolated ${\mathrm{C}}_{80}$, 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 ${\mathrm{C}}_{70}$ and give the explanation for the experimental inconsistency in structure by revealing the existence of a metastable structure. Concerning the seven isomers of ${\mathrm{C}}_{80}$ satisfying an isolated pentagon rule, it is found that the isomer with nearly ${\mathrm{D}}_2$ symmetry (${\mathrm{D}}_{2\mathrm{-}}$ ${\mathrm{C}}_{80}$) 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 ${\mathrm{D}}_{2\mathrm{-}}$ ${\mathrm{C}}_{80}$ should be detectable in an experiment. Since ${\mathrm{C}}_{80}$ 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 ${\mathrm{C}}_{80}$ ions and show the great stability of the ${\mathrm{C}}_{80}^{6\mathrm{-}}$ with nearly ${\mathit{I}}_{\mathit{h}}$ symmetry which has a larger HOMO-LUMO gap than both ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$.