TOWARDS A MICROSCOPIC APPROACH TO THE INTERMOLECULAR INTERACTION IN SOLID C-60

Although the calculation of the ground-state and thermodynamic propert ies of solid C-60 have been the subject of intense research, our under standing is still based on ad hoc models that treat phenomenologically both the Coulomb and short-range part of the interaction potential be tween C-60 molecules. These potentials do not predict well those prope rties not fitted to fix the free parameters of the model, and they als o 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 loc al-density approximation C-60 molecular charge densities, and the shor t-range part of the potential is modeled phenomenologically via Lennar d-Jones (LJ) 12-6 interactions between the centers, delocalized over t he surfaces of C-60 molecules. The regular LJ parameters sigma and eps ilon as well as multipole moments of the interaction centers distribut ion were taken to reproduce the details of the observed low-temperatur e structure. We found that the Coulomb interaction is dominated by the charge overlap between the neighboring C-60 molecules, and is much la rger than the interaction calculated using the multipole expansion of the charge densities. Contrary to common belief, this Coulomb interact ion by itself does not lead to the observed low-temperature structure. However, combined with the proposed short-range interaction, it stabi lizes Pa(3) over bar spatial structure with the correct setting angle. We make a comprehensive comparison between the wide range of experime ntal results and predictions of our, as well as previously proposed mo dels. Our results show that the proposed model has the best overall ag reement with the experimental observations in both the low- and high-t emperature phases.