To continue on your "philosophical point" Saulius, I think it is exactly what COD-TCOD-PCOD-XCODs are useful for. In my opinion, structure optimisations or predictions can also economise experimental works or developments, provided they are enough acurate, i.e. verified against existing experiments. But they could be very prospective on both sides, i.e. point to new theoretical aspects if a definitively accurate experiment does not coincide with optimised structures, or indicate ameliorations of the experiments if a valid theoretical prediction indicate whatever exp bias.

Well, both worlds have to coexist for the best of all worlds.

In MPOD, most properties do not coincide with DFT predictions, from known structures. In the case of band gaps, this latter is often refined theoretically as twice or half the observed values ... values of elastic tensors in corundum have been large question of debates in the past 100 years, both exp and theo speaking.

But I have to say, and to join Armel's point, I have always been surprised to see theo predictions corresponding to structures that were unstable under normal conditions, i.e. like if first principles were forgetting some thermodynamic variables as important as pressure, temperature ... I have also seen some band structure calculations, Lu2CuO4, an unstable compound up to 10GPa and 1500K, with no concern of any pressure, and while R2CuO4 (R= Y, and Ho to Tm) are needing such pressures.

Such extreme cases have also to be included, would it be only to reveal mistakes or unreliability of exp and theo.

daniel

Le 30/07/2014 14:00, tcod-request@lists.crystallography.net a écrit :

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Today's Topics:

1. Re: [SPAM] Re:  What structures do we accept to TCOD?
   (Saulius Gra?ulis)
2. Re: [SPAM] Re:  Re:  What structures do we accept to TCOD?
   (Stefaan Cottenier)
3. Re: What structures do we accept to TCOD? (Bj?rkman Torbj?rn)
4. Re: [SPAM] Re:  What structures do we accept to TCOD?
   (Armel le Bail)

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Message: 1 Date: Wed, 30 Jul 2014 12:12:01 +0300 From: Saulius Gra?ulis grazulis@ibt.lt To: Armel le Bail Armel.le_Bail@univ-lemans.fr, tcod@lists.crystallography.net Subject: Re: [TCOD] [SPAM] Re: What structures do we accept to TCOD? Message-ID: 53D8B6E1.3030900@ibt.lt Content-Type: text/plain; charset=windows-1252

Hello,

On 2014-07-30 01:02, Armel le Bail wrote:

...

...

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, theoreticians 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. There is here at least a point of convergence between COD and TCOD - and a serious problem with the current theoretical approach ;-).

I would take a philosophical view on this subject :). In science, we are always constrained by the limitations of our current methods, and this is true both for XRD experiments and for DFT computations. As long as these limitations are known and recorded, we can make useful predictions from the experimental and theoretical results. This is what COD and TCOD are aiming at, aren't they?

There are cases when the limitations of the methods are not exactly known, or are known only approximately. I hope that in these cases, comparing TCOD and COD would bring useful insights and help to determine those limits; and maybe to improve methods as well, both in experiment and in theoretical approaches.

I would expect that, in a "reliable prediction" limit, theoretical optimisations and experimental results must agree within error bars. If they do not, we need to investigate why. I agree that this will be a research in its own; both COD and TCOD will record self-consistent, state-of-the art results generated by both approaches (but we will want to exclude sub-standard files, that's why quality criteria are important).

If DFT computations need experimental cells for increased accuracy -- fine, COD is free and open to provide them! Moreover, in some cases COD *only* has cell constants and composition, since the experimental coordinates are behind the pay-wall of proprietary journals and databases. In these cases a DFT computation int TCOD might be the only free data set with atomic coordinates of that particular compound! If we could do this reliably on a large scale, TCOD would have enormous value as a free resource, liberating crystal structures that many people otherwise have no access to, or which we may not use as we would wish due to license limitations.

Sincerely yours, Saulius