Towards a microscopic approach to the intermolecular interaction in solid C-60

Although the calculation of the ground-state and thermodynamic properties of solid C-60 have been the subject of intense research, our understanding is still based on ad hoc models that treat phenomenologically both the Coulomb and short-range part of the interaction potential between C-60 molecules. These potentials do not predict well those properties not fitted to fix the free parameters of the model, and they also do not properly represent the Coulomb interaction between molecules. To remedy this situation, here we introduce a semiempirical model in which the Coulomb interaction is treated microscopically using the local-density approximation C-60 molecular charge densities, and the short-range part of the potential is modeled phenomenologically via Lennard-Jones (LJ) 12-6 interactions between the centers, delocalized over the surfaces of C-60 molecules. The regular LJ parameters sigma and epsilon as well as multipole moments of the interaction centers distribution were taken to reproduce the details of the observed low-temperature structure. We found that the Coulomb interaction is dominated by the charge overlap between the neighboring C-60 molecules, and is much larger than the interaction calculated using the multipole expansion of the charge densities. Contrary to common belief, this Coulomb interaction by itself does not lead to the observed low-temperature structure. However, combined with the proposed short-range interaction, it stabilizes Pa(3) over bar spatial structure with the correct setting angle. We make a comprehensive comparison between the wide range of experimental results and predictions of our, as well as previously proposed models. Our results show that the proposed model has the best overall agreement with the experimental observations in both the low- and high-temperature phases.