International Union of Crystallography, Commission on Crystallographic Apparatus. Single-crystal intensity measurement project report. II. Least-squares refinements of structural parameters

The structure factors measured by the participants in the single-crystal intensity project of the IUCr. Commission on Crystallographic Apparatus on D(+)-tartaric acid have been used in least-squares refinement of the structural parameters. The structure factors submitted by each participant were subjected to two refinements - once with heavy atoms only and once with all atoms including hydrogen. The parameters resulting from these refinements differ by magnitudes which suggest that the estimated standard deviations of the positional parameters obtained in the least-squares refinements are not infrequently a factor of about two too small and about 1/2 too small on the average. The agreement for the thermal vibration parameters may be even worse - by an additional factor of about two. These results are consistent with the indications of serious systematic errors in some of the experiments revealed in part I of this report. A modal group of six experiments with good interexperimental agreement leads to least-squares refined position parameters that are also in fair agreement; the maximum value of the ratio of the externally estimated standard deviation to the internal estimate from the least-squares refinements is about 2.5. The finding that results of possible high precision but low accuracy are not uncommon in single-crystal-structure investigations is confirmed.