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

[Saulius Gražulis]

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

-- 
Dr. Saulius Gražulis
Vilnius University Institute of Biotechnology, Graiciuno 8
LT-02241 Vilnius, Lietuva (Lithuania)
fax: (+370-5)-2602116 / phone (office): (+370-5)-2602556
mobile: (+370-684)-49802, (+370-614)-36366