  The adsorption of C60 on Si(111) has been studied by means of
first-principles density functional calculations.
  A 2x2 adatom surface reconstruction was used to simulate the terraces of the
7x7 reconstruction.
  The structure of several possible adsorption configurations was optimized
using the ab initio atomic forces, finding good candidates for two different
adsorption states observed experimentally.
  While the C60 molecule remains closely spherical, the silicon substrate
appears quite soft, especially the adatoms, which move substantially to form
extra C-Si bonds, at the expense of breaking Si-Si bonds.
  The structural relaxation has a much larger effect on the adsorption
energies, which strongly depend on the adsorption configuration, than on the
charge transfer.
