Charge density studies using CCD detectors: Oxalic acid at 100 K revisited

The charge density of oxalic acid dihydrate has been redetermined from 100 K X-ray diffraction data using a SMART CCD diffractometer. The quality of the data has been carefully analyzed and shown to be at least as good as that obtained using a point detector. The positional and thermal parameters obtained from least-squares refinement and the main features of difference density maps are comparable with those obtained by a number of laboratories as part of the IUCr electron density project. In particular, the comparison study has been completed by two multipole refinements carried out in parallel using both the current data set and that of a previous study [Stevens & Coppens (1980). Acta Cryst. B36, 1864-1876].