Intermolecular potential and the equation of state of solid C60

Abstract

First-principles total-energy calculations are performed for the low-temperature ordered phase of solid C60 in the wide range of lattice parameter. The intermolecular potential obtained from these calculations is successfully modeled in terms of the interaction between carbon atoms on different C60 molecules and the multipole Coulombic interaction between molecules. This model enables us to calculate structural and thermodynamic properties of both the orientationally ordered and disordered phases in a consistent manner. We find that the equation of state at finite temperature obtained for this potential is not in good agreement with experiments and the intermolecular potential is much shallower than that expected from the experimental heat of sublimation. These discrepancies are interpreted as arising from the limited capability of the density-functional calculations to appropriately incorporate the effect of electron correlations at large separation, which is responsible for the long-range behavior of the van der Waals interaction between molecules. In order to circumvent this difficulty we take an empirical approach to estimate the additional van der Waals interaction, which is not taken into account in the current density-functional calculations.