Calculation of optimal packing and comparison of intermolecular electrostatic interaction models in C60 crystals

Three models of intermolecular interaction potential suggested in the literature for theoretical description of polymorphous transformations in solid fullerene C-60 are compared by calculating the optimal packings of molecules. The models are characterized by effective negative charges in the middle of double bonds and opposite charges in the middle of single bonds, at the centers of five-membered rings, or an atoms (this depends on the model), For each model, two packings with different mutual orientations of molecules (I and II), which are present together in a simple cubic Pa3 phase, and several hypothetical packings obtained as potential energy minima are calculated. In the course of energy minimization, six unit cell parameters, Euler angles, and coordinates of the centers of gravity of four independent molecules are varied. If is found that all models correctly predict packing I in a global minimum. For packing II, only the "bond-bond" model is found satisfactory enough. In the case of the "bond-pentagon" model, the molecule is rotated through 25 degrees compared to the experiment. This packing is energetically less favorable than the hypothetical monoclinic packing P2(1)/n. In the case of the "bond-atom" model, the minimum of packing Il is altogether absent.