[TCOD] What structures do we accept to TCOD?

[Stefaan Cottenier]

>> 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.

Very interesting to read this statement. It shows how useful it is that 
we discuss. As to my experience, it is the other way around: theoretical 
lattice parameters are so good that there is often no need to compute 
them if you already know the experimental ones (why spending computer 
time to find the same numbers that you already knew from experiment)?

Sure, there are exceptions. And sure, it also depends on the level of 
accuracy you need: if the observable property you are interested in 
crucially depends on the 3th digit of the lattice parameter, then the 
'low quality' theoretical lattice parameters can be detrimental. But for 
the majority of cases, it simply doesn't matter whether you take 
experimental or theoretical lattice parameters.

In http://dx.doi.org/10.1080/10408436.2013.772503 , we tried to quantify 
the disagreement between experimental and theoretical cell volume for 
the PBE XC-functional. The conclusion is (Tab. 10) that PBE 
overestimates the cell volume by 3.8%, and that after correcting for 
this overestimation (if you wish to do so) there is a residual scatter 
(error bar) of 1.1 A3/atom. To give an example (see also last two lines 
of Tab. 5): this means that PBE predicts the lattice parameter of bcc-W 
with an uncertainty of 0.07 Ang. That would be a huge error bar for a 
modern diffractometer, I agree. From that point of view you are right to 
call this 'low quality'. But for many purposes that level of uncertainty 
is just fine.

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

There is another point of convergence that is imho more important: the 
crystal structure itself. >99% of all current DFT calculations start 
from complete or partial knowledge of the crystal symmetry. Doing a 
fully unbiased structure prediction by DFT, from the starting point of 
only the chemical composition, is a huge (albeit not impossible) task. 
We've done only one example of this so far 
(http://dx.doi.org/10.1039/c3ce41009a -- I love this one). It takes a 
lot of resources, though, and that's why at present everybody happily 
accepts the experimental symmetry as long as there are no indications it 
could be wrong.

This triggered another thought: isn't the information whether only 
positions were optimized and/or whether the full cell shape was 
optimized as well level-0 information? If we see only the cif without 
this information, then there is not much that can be concluded about 
such an entry. And as a corollary: doesn't that imply that there is no 
place in TCOD for DFT calculations that start from the experimental cif 
without any subsequent optimization? What do others think?

Stefaan