[TCOD] [SPAM] Re: What structures do we accept to TCOD?

[Stefaan Cottenier]

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