Role of Molecular Orientation and Surface Relaxation on Vapor Growth Shape of Molecular Crystals

Abstract

We report periodic ab initio total energy calculations to predict equilibrium morphology of molecular crystals. The approach is based on the adsorption of molecules upon the underlying crystal surfaces rather than the attachment of two-dimensional slices of thickness of interplanar distance. It takes into account the effect of molecular orientation and surface relaxation of the habit faces on the molecular attachment energy. The approach has been successfully applied to the case of urea and β-succinic acid crystals to calculate equilibrium shapes from the vapor phase. Hartree–Fock and density functional theory have been employed to obtain the atomic structure of different slices and molecular attachment energies. The relaxed morphologies obtained using the Hartree–Fock method yield good agreement with the experimental observations. Observed polar growth morphology of urea crystal has been discussed particularly in the context of different atomic environments of (111) and (111) faces.