Can Large Fullerenes Be Spherical?

MNDO geometry optimizations predict a single energy minimum for each of the Goldberg type (I_h) 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