MOLECULAR PACKING ANALYSIS - PREDICTION OF EXPERIMENTAL CRYSTAL-STRUCTURES OF BENZENE STARTING FROM UNREASONABLE INITIAL STRUCTURES

The crystal structures of benzene were analyzed to study whether it is possible to predict actual crystal structures. The minimization of en ergy and enthalpy were carried out from four types of initial structur es. These initial structures included two experimental structures of a ctual phases and two unreasonable structures created artificially. Fro m an artificial initial structure, we were able to obtain a similar lo cal minimum structure to an observed low pressure phase by energy mini mization at zero pressure. The enthalpy minimum structures at 2.5 GPa, which were obtained from two artificial initial structures, were in g ood agreement with the observed structure of a high pressure phase. We made sure that the corresponding structure of each observed phase was the most stable structure at each pressure but was the metastable str ucture at the other pressure. The isostress-isoenthalpic molecular dyn amics (MD) calculations were carried out starting from each energy min imum structure at two temperatures (147 K, 242 K) lower than the melti ng point. The crystal structure corresponding to the observed low pres sure phase (the energy global minimum) was stable in the trajectory of MD calculations. In the MD calculation starting from local minimum st ructures, the unit cell deformed greatly and the positions of benzene molecules in the unit cell shifted far from their initial positions.