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

Tue Feb 12 22:28:49 CET 2019

Hello Nicola,

I am aware of many of the items you have stated. If you could show me a calculation of the Elastic Tensor with ESPRESSO for Si I would be more inclined to believe that ESPRESSO is not at fault.  I tested different pseudopotentials, it didn't help ESPRESSO's case. I did converge to what I thought was an appropriate cutoff and kpoint mesh (I believe I used 30Ry and 11x11x11, comparable to materials project parameters used in VASP).

Again, perhaps just a thought from me, it could be due to breaking the symmetry of the crystal as is the case when you apply a deformation.

I will get back to you as soon as I can with more information. On the meantime, it might be nice if one of you can do a calculation with ElaStic to what you may believe would be overconverged values.

P.S. Unless I am missing the point, I fail to see how the link you showed me affects the elastic tensor.
________________________________
From: Nicola Marzari <nicola.marzari at epfl.ch>
Sent: Tuesday, February 12, 2019 9:18 PM
To: General discussion list for Quantum ESPRESSO developers; Christopher Pashartis (----)
Subject: Re: [QE-developers] Significant Deviations from Elastic Tensors from existing codes

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/>
>
> Kapeldreef 75I3001 LeuvenIBelgium
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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
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