We discuss the results of large-scale density functional calculations for (root 7x root7) benzene adsorption on the ferromagnetic substrate Ni{111}. Adsorption at the bridge site, with C-C bonds parallel to the [(2) over bar 11] direction, is found to be energetically preferred over the [(1) over bar 10]-oriented hcp site suggested by electron diffraction experiments. We discuss the roles of intermolecular forces and anisotropic molecular vibrations in accounting for this apparent discrepancy. Furthermore, our analysis of charge and spin densities clarifies the relationship between preferred adsorption sites and the symmetry of frontier molecular orbitals. (C) 2001 American Institute of Physics.