Dear all,
It seems that I forgot to add the list to recipients... Armel, do read on, I added some things.
I would say no, for two reasons. The first is practical: It would require the possibility to somehow automatically determine a suitable experimental reference structure. I don't think that this is possible, since the criteria go way beyond "purely crystallographic" considerations (Fobs et cetera). You'd need to know that the sample is actually pure enough (in DFT we always have perfect samples...), precisely how were samples prepared, what the temperature effects are like and so on...
The second is from basic principles. I think that would be going into data analysis, whereas the point of TCOD is just storage of data, with quality control done within DFT itself (to the extent that this is possible). Requiring a comparison to experimental data to me makes about as much sense as requiring supporting DFT calculations for inclusion of experimentally determined structures in COD. All the arguments against such a procedure apply equally well in both directions.
There is also the problem that, in terms of Saulius checkpoint list, we also have the possibility b''') Neither experiments or theory have any problems, but you are comparing apples and bananas due to too little information being available on either side. For example, a DFT calculation (which assumes a temperature of 0K) would tell you that the experimentally observed bcc titanium is nonsense, whereas it is in fact a high temperature phase that is stabilized by phonon-phonon interactions. If temperature data would be missing (or, worse, being set to some default ~300K), this would generate confusion. The only time that this is straightforward is in cases where a lot is known and everyone knows the correct answer anyway, but then these are the cases we are trying to venture beyond...
It would of course be very nice to at some point have a "gold standard" database, containing only structures that are exceedingly well known and understood both from the points of theory and experiment, but I don't think that either COD or TCOD should be that database.
Regards, Torbjörn
--- Torbjörn Björkman, PhD COMP, Aalto University School of Science Espoo, Finland
________________________________________ Från: tcod-bounces@lists.crystallography.net [tcod-bounces@lists.crystallography.net] för Armel le Bail [Armel.le_Bail@univ-lemans.fr] Skickat: den 30 juli 2014 01:02 Till: tcod@lists.crystallography.net Ämne: Re: [TCOD] [SPAM] Re: What structures do we accept to TCOD?
Hi,
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. There is here at least a point of convergence between COD and TCOD - and a serious problem with the current theoretical approach ;-).
Best,
Armel
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Hi,
So, finally, it is more comparing (often) a DFT structure at OK but having the cell parameters from an experimental structure at 293K to that experimental structure rather than comparing apples and bananas.
Would be time to make the effort to have DFT results at any temperature including the phase transition predictions. I am sure you also dream of that...
Best, Armel
So, finally, it is more comparing (often) a DFT structure at OK but having the cell parameters from an experimental structure at 293K to that experimental structure rather than comparing apples and bananas.
:-) That's the nicest tongue-in-cheek statement I've read in the past few months.
Would be time to make the effort to have DFT results at any temperature including the phase transition predictions. I am sure you also dream of that...
To quite an extent, that is possible. Instead of having only the total internal energy U (which DFT gives you), one needs also the temperature dependent entropy S(T). In that way, you have the temperature dependent free energy F(T)=U-T*S(T). If you compute F(T) for every relevant phase, you can determine the phase transition temperatures.
The question is how to compute S(T). Phonons give the largest contribution to S(T), hence you need a full phonon spectrum -- which is surely doable, but time-consuming. Phonons (=harmonic) are a good description only at not too high temperatures, hence you need anharmonic contributions if you are interested is the high temperature range -- again more time-consuming. And if you want/need to include electronic or magnetic contributions to S(T), again more efforts are needed.
So, it's possible, but you need a sufficiently interesting question to justify the computing effort. It's not yet something that can be routinely done, and therefore it is not yet at the stage where it becomes useful to build a database of results.
Stefaan
tcod@lists.crystallography.net
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Armel le Bail -
Björkman Torbjörn -
Stefaan Cottenier