Density functional calculations of the structure of crystalline urea under high pressure

The geometry of the urea crystal is optimized for several different pressures using a pseudopotential density functional (DFT) approach with a plane-wave basis set and the Ceperley–Alder local density correlation potential. All the optimized intra- and intermolecular geometrical parameters, as well as the lattice vectors, are found to be in good agreement with experiment. Under high pressures, the crystal changes anisotropically; the lattice constants a and b reduce by almost 10 times more with the pressure than the constant c. Changes of the geometrical parameters within the molecule as a function of the external pressure are found to be in good agreement with the donor–acceptor theory.