Molecular packing analysis: prediction of experimental crystal structures of benzene starting from unreasonable initial structures

Abstract

The crystal structures of benzene were analyzed to study whether it is possible to predict actual crystal structures. The minimization of energy and enthalpy were carried out from four types of initial structures. These initial structures included two experimental structures of actual phases and two unreasonable structures created artificially. From an artificial initial structure, we were able to obtain a similar local minimum structure to an observed low pressure phase by energy minimization at zero pressure. The enthalpy minimum structures at 2.5 GPa, which were obtained from two artificial initial structures, were in good 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 structure at the other pressure. The isostress-isoenthalpic molecular dynamics (MD) calculations were carried out starting from each energy minimum structure at two temperatures (147 K, 242 K) lower than the melting point. The crystal structure corresponding to the observed low pressure phase (the energy global minimum) was stable in the trajectory of MD calculations. In the MD calculation starting from local minimum structures, the unit cell deformed greatly and the positions of benzene molecules in the unit cell shifted far from their initial positions.