Applications of DL_POLY and DL_MULTI to organic molecular crystals

Molecular dynamics (MD) simulations are capable of giving considerable insight into the polymorphism of organic molecules, a problem of major concern to the pharmaceutical and other speciality chemicals industries. We illustrate some of the challenges involved in small organic systems, which have complex solid-state phase behaviour, including characterizing rotationally disordered phases, modelling polymorphs with very different hydrogen bonding motifs and explaining the solvent dependence of a polymorphic system. Simulating the dynamics within the organic solid state can be very demanding of the model for the weak forces between the molecules. This has led to the development of DL_MULTI so that a distributed multipole electrostatic model can be used to describe the orientation dependence of hydrogen bonding and pi-pi stacking more realistically. Once a simulation is correctly reproducing the known crystal structures, there are also considerable system-specific challenges in extracting novel insights from the MD simulations.