INTRAMOLECULAR-FORCE-CONSTANT MODEL FOR C-60

A least-squares fit of a short-range-force-constant model to the 14 Raman and infrared active normal-mode frequencies of C60 has been obtained. The fit is better than 3.8% (in frequency) for any of these modes. The model, which has seven adjustable parameters, consists of two- and three-body valence-force-field terms and a four-body potential-energy term taken to be proportional to the square of the deviation in the volume of a tetrahedron formed by an atom and its three nearest neighbors. In its effect on the atomic vibrations, the latter term is closely related to the out-of-plane, or puckering, force-constant term proposed several years ago for graphite. The resulting parameters of the model are also quantitatively similar to those for that model of graphite. The full frequency spectrum based on the model compares reasonably well with recent inelastic neutron-scattering measurements. In addition, first-principles (local-density-approximation) calculations of the A(u)-mode frequency have recently become available; the model yields 1012 cm-1 for this mode, which is 16+/-3% higher than the first-principles result.