Electron density study of urea using TDS-corrected X-ray diffraction data:quantitative comparison of experimental and theoretical results

The electron-density distribution in urea, CO(NH2)(2), was studied by high- precision single-crystal X-ray diffraction analysis at 148 (1) K. An experi mental correction for TDS was applied to the X-ray intensities. R-merge(F-2 ) = 0.015. The displacement parameters agree quite well with results from n eutron diffraction. The deformation density was obtained by refinement of 1 45 unique low-order reflections with the Hansen & Coppens [Acta Cryst. (197 8), A34, 909-921] multipole model, resulting in R = 0.008, wR = 0.011 and S = 1.09. Orbital calculations were carried out applying different potential s to account for correlation and exchange: Hartree-Fock (HF), density-funct ional theory/local density approximation (DFT/LDA) and density-functional t heory/generalized gradient approximation (DFT/GGA). Extensive comparisons o f the deformation densities and structure factors were made between the res ults of the various calculations and the outcome of the refinement. The agr eement between the experimental and theoretical results is excellent, judge d by the deformation density and the structure factors [wR(HF) = 0.023, wR( DFT) = 0.019] and fair with respect to the results of a topological analysi s. Density-functional calculations seem to yield slightly better results th an Hartree-Fock calculations.