[Wien] Inconsistency between energy and forces ?

[Stefaan Cottenier]

> Just a guess: Please make a short test with LDA (5) instead of GGA.
> I'd like to see if the total energy differences between the two
> structures comes from GGA-grids.

Indeed, with LDA the energies are only 0.0008 apart. And also the forces are
almost identical now in the small and the large cell. This feels like
magic...!

Is it possible to explain the background of this? And why is the problem
apparantly related to particular cell symmetries (and not to particular
atoms, or atom-atom distances, ...)? Is it known which space groups are
'vulnerable' for GGA? Can it be corrected with e.g. a larger NPT? ... ?

Thanks,
Stefaan