Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > 2σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.There is a pseudo inversion at (0.75070 0.50000 0.62576), but it does seem as if the space group really is P21. This came as a great surprise because there seems to be no obvious reason why this structure would be non-centrosymmetric. All indications are that the crystals themselves are non even inversion twins because the Flack (and Hooft 'y') parameters are both zero within a couple of SUs. Although these SUs are a bit larger than the recommendation suggested by Flack. Further tests with various procedures in PLATON (including ADDSYM) suggest "No Obvious Spacegroup Change Needed/Suggested", but the checkCIF implementation of ADDSYM does suggest "ADDSYM Detects Additional (Pseudo) Symm. Elem··· m", but on inspection the structure does not seem to have any kind of mirror plane. Further, the checkCIF implementation of ADDSYM/MISSYM suggests "Potential lattice centering or halving", but again, on inspection of the model and the diffraction data this does not appear to be the case.