[QE-developers] Significant Deviations from Elastic Tensors from existing codes

[Nicola Marzari]

Dear Christopher,


thanks for reporting, but no bug - QE and VASP (if you use GW-ready 
pseudo) have actually the best agreement of bulk moduli with all-
electron codes: https://molmod.ugent.be/deltacodesdft

Discrepancies between codes are due to different pseudopotentials or the 
numerically details needed to get a well converged calculation - not 
trivial, at al:  it depends if you are fitting the energy or the stress,
how much strain you are using, superconvergence on cutoffs, kpoints and 
selfconsistency - hence why most values in the literature are poorly 
converged, and properly done calculations, as those in the link above,
agree very closely.

				nicola




On 12/02/2019 07:49, Christopher Pashartis (----) wrote:
> Dear Developer(s),
> 
> I have encountered what would appear to be a serious bug in your code. I 
> have been trying to tabulate some elastic constants for Si and Cu only 
> to realize that your code has some issues with some components of the 
> elastic tensor.
> 
> Namely, the c44 component of Si and c11 component of Cu. All my other 
> parameters are within a reasonable range of existing VASP calculations.
> 
> For example Si (in GPa):
> Component
> 	Materials Project (VASP)
> 	ESPRESSO with the same routine as Materials Project (I did these calcs)
> c11
> 	144
> 	156
> c12
> 	53
> 	62
> c44
> 	75
> 	104
> 
> I have converged my cutoff parameters and kpoint mesh. I also tried 
> fiddling with the tolerances on energy and forces./*All of these are 
> converged in the values I show you above*/​.It would seem both your 
> energies and forces are miscalculated in certain deformation directions 
> (and no I did not apply too much of a deformation). I am fairly certain 
> that these issues have nothing to do with my code as I also implemented 
> it for CP2k and the results were within acceptable margins of existing code.
> 
> If you require any more information to assist you in this effort, let me 
> know.
> 
> *CHRISTOPHER PASHARTIS*
> 
> PhD Student in Computational Materials
> 
> christopher.pashartis at imec.be<mailto:christopher.pashartis at imec.be>I_www.imec.be_<http://www.imec.be/>
> 
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-- 
----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, EPFL
http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project