A b i n i t i o approach to molecular crystals: A periodic Hartree–Fock study of crystalline urea

The electronic structure of crystalline urea (two molecules, 16 atoms per unit cell) is investigated at an ab initio level with CRYSTAL, a Hartree–Fock linear combination of atomic orbitals (LCAO) program for periodic systems. The influence of the basis set and of the computational parameters which control the treatment of the Coulomb and exchange series and the reciprocal space integration is documented; results include total and interaction energy, Mulliken analysis data and interaction (solid minus molecules) density maps, band structure, and density of states. The crystal field modifies the electronic structure of the isolated molecule, the main effect being an increase in the ionicity of bonds. The interaction energy obtained with a 6-21** basis set is 28 kcal/mol, (16 kcal/mol after a correction of the basis set superposition error by using the counterpoise method) to be compared with 21±0.5 kcal/mol from experiment. This preliminary application shows that accurate ab initio calculations of hydrogen bonded molecular crystals with a relatively small unit cell (less than 30 atoms/cell) are now within reach.