[TCOD] cell parameters - or others

Thu Jul 31 14:55:31 UTC 2014

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 at lists.crystallography.net a écrit :
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>     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)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Wed, 30 Jul 2014 12:12:01 +0300
> From: Saulius Gra?ulis <grazulis at ibt.lt>
> To: Armel le Bail <Armel.le_Bail at univ-lemans.fr>,
> 	tcod at lists.crystallography.net
> Subject: Re: [TCOD] [SPAM] Re:  What structures do we accept to TCOD?
> Message-ID: <53D8B6E1.3030900 at 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
>

-- 

----------------------------------------------------------------------
Daniel Chateigner
Professeur, Université de Caen Basse-Normandie
http://www.ecole.ensicaen.fr/~chateign/danielc/
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