Theoretical Studies on the Smallest Fullerene: from Monomer to Oligomers and Solid States

Hybrid B3LYP and density‐functional‐based tight‐binding (DFTB) computations on the solid‐state structures and electronic properties of the C 20 fullerene monomer and oligomers are reported. C 20 cages with C 2 , C 2 h , C i , D 3 d , and D 2 h symmetries have similar energies and geometries. Release of the very high C 20 strain is, in theory, responsible for the ready oligomerization and the formation of different solid phases. Open [2+2] bonding is preferred both in the oligomers and in the infinite one‐dimensional solids; the latter may exhibit metallic character. Two types of three‐dimensional solids, the open [2+2] simple cubic and the body‐centered cubic ( bcc ) forms, are proposed. The energy of the latter is lower due to the better oligomer bonding. The open [2+2] simple cubic solid should be a conductor, whereas the bcc solids are insulators. The most stable three‐dimensional solid‐state structure, an anisotropically compressed form of the bcc solid, has a HOMO–LUMO gap of approximately 2 eV and a larger binding energy than that of the proposed C 36 solid.