My point is: the task of comparing experiment and theory (i.e. comparing COD and TCOD) is a research task in its own right.
To my knowledge, theoricians are aware about the low quality of the cell parameters derived from their DFT optimizations to the point that they prefer to fix them to the experimental values when available.
Very interesting to read this statement. It shows how useful it is that we discuss. As to my experience, it is the other way around: theoretical lattice parameters are so good that there is often no need to compute them if you already know the experimental ones (why spending computer time to find the same numbers that you already knew from experiment)?
Sure, there are exceptions. And sure, it also depends on the level of accuracy you need: if the observable property you are interested in crucially depends on the 3th digit of the lattice parameter, then the 'low quality' theoretical lattice parameters can be detrimental. But for the majority of cases, it simply doesn't matter whether you take experimental or theoretical lattice parameters.
In http://dx.doi.org/10.1080/10408436.2013.772503 , we tried to quantify the disagreement between experimental and theoretical cell volume for the PBE XC-functional. The conclusion is (Tab. 10) that PBE overestimates the cell volume by 3.8%, and that after correcting for this overestimation (if you wish to do so) there is a residual scatter (error bar) of 1.1 A3/atom. To give an example (see also last two lines of Tab. 5): this means that PBE predicts the lattice parameter of bcc-W with an uncertainty of 0.07 Ang. That would be a huge error bar for a modern diffractometer, I agree. From that point of view you are right to call this 'low quality'. But for many purposes that level of uncertainty is just fine.
There is here at least a point of convergence between COD and TCOD - and a serious problem with the current theoretical approach ;-).
There is another point of convergence that is imho more important: the crystal structure itself. >99% of all current DFT calculations start from complete or partial knowledge of the crystal symmetry. Doing a fully unbiased structure prediction by DFT, from the starting point of only the chemical composition, is a huge (albeit not impossible) task. We've done only one example of this so far (http://dx.doi.org/10.1039/c3ce41009a -- I love this one). It takes a lot of resources, though, and that's why at present everybody happily accepts the experimental symmetry as long as there are no indications it could be wrong.
This triggered another thought: isn't the information whether only positions were optimized and/or whether the full cell shape was optimized as well level-0 information? If we see only the cif without this information, then there is not much that can be concluded about such an entry. And as a corollary: doesn't that imply that there is no place in TCOD for DFT calculations that start from the experimental cif without any subsequent optimization? What do others think?
Stefaan