Can Large Fullerenes Be Spherical?

MNDO geometry optimizations predict a single energy minimum for each of the Goldberg type (Ih) fullerenes C180, (C240, C540, and C960 which corresponds to an icosahedrally shaped structure with strong curvature at the 12 pentagons and nearly planar segments composed of hexagons. Constrained geometry optimizations preserving a spherical shape lead to considerably larger energies and show that an evenly distributed curvature is strongly disfavored. The results are confirmed quantitatively by ab initio SCF calculations for C180 and C240 employing a split valence basis set, but contrast the conclusions from a previous density functional study. The observed trends are discussed on the basis of curvature-corrected Huckel calculations and simple force field estimates