Prediction of fullerene packing in C60 and C70 crystals

A force field recently developed for sp2 carbon atoms is used here to predict the crystal structures and cohesive energies for close-packed crystals of C60 and C70. It is predicted, and confirmed from calculations, that for C60 fcc packing is more stable than hcp by 0.90 kcal/mol, whereas for C70 hcp is more stable than fcc by 0.35 kcal/mol. The cubic structure of C60 undergoes an orthorhombic distortion to space group Cmca at 0 K. At higher temperatures there is rapid reorientation (but not free rotation) of C60 molecules, suggesting that above about 200 K a phase transition occurs to an orientationally distorted, fcc structure.