COMPUTATIONAL INVESTIGATION OF SURFACE STRUCTURAL RELAXATION IN CRYSTALLINE UREA

The effects of surface structural relaxation on the energetic and morp hological properties of crystalline urea have been investigated. Speci fically, the {001}, {110}, {200}, {101}, {(111) over bar} and {(111) o ver bar} surfaces have been considered. Upon relaxation of the cleaved surfaces, significant perturbations in the positions and internal geo metries of the molecules (compared with the corresponding positions in the bulk crystal structure) are observed. Interestingly, the {(111) o ver bar} surface has a lower surface energy than the {111} surface, an d thus a polar morphology is predicted. The predicted crystal shape is compared with that observed experimentally (for crystals grown by dep osition from the vapour phase), and reasons for discrepancies between the calculated and experimental morphologies are discussed.