Molecular crystal global phase diagrams. III. Sufficient parameter space determination

Abstract

In previous parts of this series [Mettes et al. (2004). Acta Cryst. A60, 621-636; McClurg & Keith (2010). Acta Cryst. A66, 38-49] a method for constructing global phase diagrams (GPDs) for molecular crystals was developed and the method was applied to single-component ordered crystal structures of tetrahedral molecules. GPDs are useful for visualizing what types of crystal structures a given molecule may assume depending on molecular form/interaction. Their construction uses group-theoretical methods which enumerate all possible symmetry breakings during a statistical mechanical high-to-low temperature search. In this work these results are expanded upon by outlining a method to determine a sufficiently rich parameter space to represent the experimentally observed crystal structures in a data set derived from the Cambridge Structural Database. This is significant because previous work (Mettes et al., 2004) did not specify the number of parameters needed for GPDs. Although there are suggestions in the literature that thousands of parameters are required to adequately describe tetrahedral molecule intermolecular potentials, it is found that 15 parameters are sufficient to represent the structures of the test data. The origin of this difference and its implications for determining GPD parameter values from a more detailed intermolecular potential and for interpreting GPD parameter values are discussed.