The low-frequency regions of the Inelastic Neutron Scattering spectra of K 3 C 60 , Rb 3 C 60 , and Rb 6 C 60 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 K 3 C 60 and Rb 6 C 60 , but systematically underestimates the location of some bands, especially for Rb 3 C 60 . Analysis of the results suggests that the inaccuracy is caused by the atom-atom van der Waals interactions.