Global analysis of crystal energy landscapes: applying the threshold algorithm to molecular crystal structures

We describe the implementation of the Monte Carlo threshold algorithm for molecular crystals as a method to provide an estimate of the energy barriers separating crystal structures. By sampling the local energy minima accessible from multiple starting structures, the simulations yield a global picture of the crystal energy landscapes. This provides valuable information on the depth of the energy minima associated with crystal structures and adds to the information available from crystal structure prediction methods that are used for anticipating polymorphism. We present results from applying the threshold algorithm to four polymorphic organic molecular crystals, examine the influence of applying space group symmetry constraints during the simulations, and discuss the relationship between the structure of the energy landscape and the intermolecular interactions present in the crystals.