CALCULATION OF OPTIMAL PACKING AND COMPARISON OF INTERMOLECULAR ELECTROSTATIC INTERACTION MODELS IN C-60 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 packi ngs of molecules. The models are characterized by effective negative c harges in the middle of double bonds and opposite charges in the middl e of single bonds, at the centers of five-membered rings, or an atoms (this depends on the model), For each model, two packings with differe nt mutual orientations of molecules (I and II), which are present toge ther in a simple cubic Pa3 phase, and several hypothetical packings ob tained as potential energy minima are calculated. In the course of ene rgy minimization, six unit cell parameters, Euler angles, and coordina tes 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 satisfa ctory enough. In the case of the ''bond-pentagon'' model, the molecule is rotated through 25 degrees compared to the experiment. This packin g is energetically less favorable than the hypothetical monoclinic pac king P2(1)/n. In the case of the ''bond-atom'' model, the minimum of p acking Il is altogether absent.