Electron-density-based calculations of intermolecular energy: case of urea

The intermolecular interaction energy in crystalline urea has been calculat ed both from diffraction data and from the Hartree-Fock crystalline electro n-density distribution, using a modified atom-atom approximation scheme. Th e electrostatic part of this energy has been calculated from the atomic mul tipole moments, obtained by adjustment of the multipole model to experiment al X-ray and to theoretical Hartree-Fock structure amplitudes. To obtain th e induction energy, multipole moments were calculated from structure amplit udes for the crystalline electron density and from those that refer to the electron density of a superposition of isolated molecules. This worked well for the calculation of the interaction energy from Hartree-Fock data (6% d ifference from the sublimation-energy value), but not for the interaction e nergy from experimental data, where the moments of the superposition have t o come from Hartree-Fock calculations: the two sets of multipole moments ar e far too different. The uncertainty of the phases of the structure amplitu des, combined with systematic errors in the theoretical data and noise in t he experimental values, may account for the discrepancies. The nature of th e different contributions to intermolecular interactions for urea is examin ed.