Intercage dynamics of C60 in doped crystals

The low-frequency regions of the Inelastic Neutron Scattering spectra of K3C60, Rb3C60, and Rb6C60 are modelled by atomistic Molecular Dynamics simulations followed by Fourier Transform. The use of a standard force field for the interatomic interactions, together with the simple hypothesis that the charges are homogeneously distributed over the cage, obtains a reasonable agreement with the experiments for the spectra of K3C60 and Rb6C60, but systematically underestimates the location of some bands, especially for Rb3C60. Analysis of the results suggests that the inaccuracy is caused by the atom-atom van der Waals interactions.