A new method of probing surface-surface interactions and calculating attach
ment 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 ca
pabilities 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 wit
h accurate anisotropic atom-atom intermolecular potentials, including distr
ibuted-multipole electrostatic models. Properties pertinent to the morpholo
gy and agglomeration of urea crystals are reported, including surface relax
ation, attachment energies and surface energies, solvent and solute binding
energies, and the inter-surface interaction energy. We correctly predict t
he two major forms {110} and {001} of vapour-grown urea crystals, including
an observed aspect ratio. The polar cap facets of the crystals probably ar
ise from the unusually large relaxation of a polar {111} surface which prov
ides a further kinetic barrier to growth. A comparison of the binding energ
ies of water and urea molecules to the different surfaces shows that the gr
owth 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 domina
nt (110) and (001) crystal faces has also been calculated, and was found to
be only about 20% smaller than the interaction between (110) surfaces.