Neutron diffraction data were collected on urea at 60, 123 and 293 Κ, and the nuclear parameters were determined [/? (F) = 0.034,0.033 and 0.043 respectively]. The parameters at 293 Κ are compared with the neutron parameters of Pryor and Sanger by means of half normal plots and χ 2 tests, and the low-temperature parameters with the ls 2 -core parameters which were determined by Mullen and Hellner with X-ray data. Significant differences of0.0030 Â, on average, were found between the t/ 33 components from our low-temperature determination and from the Is 2 -core refinement. X-Ν and X-X(1 s 2 ) maps are calculated for about 100 Κ with two different scale factors and, after an assessment of all parameters, a "best" experimental deformation density is calculated. The effect of large librations of the Ν atom about the C-O axis (4.2 rms degrees) on the density distribution, and the effect of using not fully correct phases are discussed. The errors in the density distribution which are caused by errors in the X-ray data, in the scale factor and in the neutron parameters, are assessed to be 0.07 e -3 at the bond peaks, and 0.45 eA" 3 at the nuclear positions. The "best" experimental deformation density has higher bond peaks (except for the C-O bond) than a (published) theoretical difference density (4-31G basis), but lower peaks in the lone-pair regions of the oxygen atom and no polarization minimum at the oxygen atom.

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