The electron density of a-Oxalic acid dihydrate at 100 K by X-ray diffraction: a contribution to the IUCr Commission on Charge, Spin and Momentum Densities project on the accurate determination of electron densities

The results of the determination of the electron density of u-oxalic acid dihydrate at 100 K by means of X-ray diffraction are reported as part of the project on the accurate determination of electron densities which was initiated by the International Union of Crystallography. An extensive data set, consisting of all reflexions in the whole reciprocal sphere up to sin 0/2 =1.3 ,/~-l, was measured. Most of the structural parameters and experimental deformation densities compare well with the findings of an earlier published study [Stevens & Coppens (1980). Acta Cryst. B36, 1864-1876]; thermal parameters, however, differ significantly. Application of a correction for thermal diffuse scattering increased the vibrational parameters by 12%. The polarization of the water O lone-pair density, found by Stevens & Coppens (1980), is firmly confirmed by this study.