Dear Bhuv
> pattern and I have corrected possible graininess with spherical [...] > The data I have collected is on a image plate (only one frame).
Not sure I understand? If you have a 2D image showing powder rings then you should have some very good ideas about the level of granularity or texture in the sample. Just look for the variation in intensity versus azimuth? Did you mean a one dimensional image plate?
One way to reduce "graininess" if you have a mixture of grains and fine powder: take the median when integrating around the rings instead of the mean. If you only have grains and no continuous rings then better to do the single crystal experiment...
> In such a case, is it possible to give (any meaningful) Standard > Uncertainties for the atom positions, (also) given the fact that the > atoms are not refined independently?
Some comments:
1) "Standard Uncertainty" is a defined statistical quantity. Always theoretically possible to derive it from a least squares refinement. It should always represent what it is defined to represent. Rarely what you want to know ;-)
2) If the cell parameters a,b,c of a cubic crystal are constrained to be equal I assume the value and esd is the same for all three, and that they are 100% correlated. That they have not been refined independently would be a relief (equivalent feats are sometimes attempted via Rietveld).
3) For your constrained positions the esd from the refinement may reflect the esd on the position and orientation rather the individual atom position. It just means there are very high correlations being hidden by the constraints (Z-matrix or otherwise).
4) Replace "Z-matrix constraint" with "symmetry operator constraint" and then decide if you could bring yourself to list atomic positions and esds in a lower space group than the one you used for refinement. (eg: for comparing structures above and below a phase transition)
So "yes" it seems possible to give "meaningful" standard uncertanties for the atomic positions, provided you mention the constraints and restraints used. Although they are not interesting, they are considerably more meaningful than Rwp in terms of evaluating the structure!
Good luck,
Jon
