Stiffness of a solid composed of ${\mathrm{C}}_{60}$ clusters

We calculate the equilibrium structure of fullerite, a solid composed of ${\mathrm{C}}_{60}$ clusters, as a function of external pressure. We find that at zero pressure, carbon atoms in neighboring ‘‘fullerene’’ clusters are no closer than 2.65 Å apart and interact by pairwise van der Waals forces. At increasing pressures, we observe a gradual transition to a hard-core repulsion between neighboring clusters. Only at high pressures, beyond ≊70 GPa, does the bulk modulus of fullerite exceed that of diamond.