Dear Armel,
However, I have almost never seen the comparison of the experimental data to the intensities calculated from the optimized atomic coordinates... I have especially in mind a "simple" example, calcium carbonate, vaterite form, for which dozens of optimized models were proposed, without even one comparison to the data...
I don't think this is really rare. But often there is simply no need to do it. One case in which I was somewhat involved makes such an explicit comparison in figs. 3-4-5: http://dx.doi.org/10.1103/PhysRevB.84.054110 . Another example that comes to my mind is a structure prediction problem by Oganov (fig. 2 in http://dx.doi.org/10.1016/j.epsl.2005.10.014). And I remember a conference presentation by Chris Wolverton where he does DFT structure prediction not by optimizing total energy but by minimizing the difference between the experimental and DFT-based powder spectrum.
So the question is : In case of the existence of an experimental dataset, should a comparison observed/calculated be requested (classical R values but with fixed coordinates) ?
According to me: no. Let me slightly idealize, and assume that for every entry in TCOD the numerics are done right. In that case, all entries would contain the exact/unique/precise prediction for a particular crystal *at the given level of theory*. Everyone working at the same level of theory (e.g. using the same XC-functional within DFT) would make essentially the same prediction. That may or may not agree with the experimental structure, either because the experiment is 'wrong' (e.g. a small contamination that triggers a different crystal structure than for the pure material that has been calculated) or because the level of theory is insufficient (there is no XC-functional known that is 100% correct for all classes of materials).
My point is: the task of comparing experiment and theory (i.e. comparing COD and TCOD) is a research task in its own right. It should be performed by the users of these databases. For TCOD as a computational database, comparison to the alien world of experiments is not the thing to do. It would be similar to allowing into COD only those structures that have survived a DFT check ;-)
Best, Stefaan