VAN-DER-WAALS ATTRACTION BETWEEN 2 C60 FULLERENE MOLECULES AND PHYSICAL ADSORPTION OF C60 ON GRAPHITE AND OTHER SUBSTRATES

The van der Waals interaction between two C60 fullerene molecules is computed using a discrete-dipole formalism in which C60 is viewed as a rigid cluster of 60 polarizable, interacting carbon atoms. The dispersion energy is obtained from the shift in zero-point energy of the ground-state dipolar fluctuations. The intermolecular attraction potential deduced from this approach is compared with the Girifalco potential [J. Chem. Phys. %, 858 (1992)] which includes only averaged, pairwise additive interactions. The results are also compared with the potential of two continuum dielectric shells of finite thickness [Ph. Lambin et al., Phys. Rev. B 46, 1794 (1992)]. The dispersion energy between C60 and graphite is also computed with the same discrete-dipole molecular model and by treating the substrate as a continuous dielectric medium. The adsorption energy is compared with the result of summing discrete 1/r6 C-C interactions. Similar calculations are presented for a C60 molecule on semiconductors (Ge, Si, and GaAs) and insulators (LiF and MgO). Lastly, the substrate-mediated dispersion energy between two adsorbed molecules is discussed. The calculations include all orders of dipole-dipole interactions between the molecules and with the substrate. The comparison with pairwise additive dispersion energies brings out the non-negligible role of many-body contributions.