Developments in computational studies of crystallization and morphology applied to urea

A new method of probing surface-surface interactions and calculating attachment energies for morphology predictions, based on the interactions between an infinite surface and a thin finite slice (a nano-crystallite), has been implemented in the Orient program package. This, together with existing capabilities for studying 2D periodic surface adlayers, or isolated molecular clusters on a surface, enables a wide range of complementary calculations to be performed to study crystallization phenomena of organic molecules with accurate anisotropic atom-atom intermolecular potentials, including distributed-multipole electrostatic models. Properties pertinent to the morphology and agglomeration of urea crystals are reported, including surface relaxation, attachment energies and surface energies, solvent and solute binding energies, and the inter-surface interaction energy. We correctly predict the two major forms {110} and {001} of vapour-grown urea crystals, including an observed aspect ratio. The polar cap facets of the crystals probably arise from the unusually large relaxation of a polar {111} surface which provides a further kinetic barrier to growth. A comparison of the binding energies of water and urea molecules to the different surfaces shows that the growth of the {110} surfaces will be particularly impeded by the presence of water. This rationalizes the increased morphological dominance of this face in crystals grown from solution. The interfacial energy between the dominant (110) and (001) crystal faces has also been calculated, and was found to be only about 20% smaller than the interaction between (110) surfaces.