[QE-users] Choosing convergence criteria for vc-relax

Mon May 20 20:21:22 CEST 2019

Dear Nicola,
Thank you very much!
Regards,
Michal

On Mon, 20 May 2019 at 20:04, Nicola Marzari <nicola.marzari at epfl.ch> wrote:

>
>
> Dear Michal,
>
>
> it depends on how smooth is the dependences of the forces on the cutoff,
> so one should make an informed decision depending on the
> pseudopotentials used. We e.g. look at phonon frequencies as a function
> of cutoff, and if you look at silicon, here below, even if we didn't
> test below 30Ry, I would bet that at 20 or even 15Ry forces would be
> reasonable:
>
>
> https://www.materialscloud.org/discover/data/discover/sssp/convergences_efficiency/Si_8.0_conv_patt.png
>
> If you try Ga, you can see that the phonons (the solid line) do not
> particularly become worse in going down from 70Ry to 45Ry, but then
> progressively worsen:
>
>
> https://www.materialscloud.org/discover/data/discover/sssp/convergences_efficiency/Ga_8.0_conv_patt.png
>
> Still, for many pseudopotentials a decrease in cutoff doesn't worsen the
> forces particularly, but there is no 2/3 or 1/2 rule - one just need to
> examine the tables above.
>
> For ecutrho it might be a bit easier, and if you are not having
> magnetism often one can use a dual (the ration of ecutrho/ecutwfc)
> equalt to 3, or even 2, for normconserving, and 6 or even 4 for ultrasoft.
>
> All to be tested...
>
>                                 nicola
>
>
> On 20/05/2019 13:47, Michal Krompiec wrote:
> > Dear Nicola,
> > I am looking forward to reading the "short thing on this"!
> > Can you comment on the widespread (?) practice of using much lower than
> > recommended (say, 2/3 or 1/2) ecutwfc for geometry optimization until a
> > minimum is found, and then increasing cutoffs until convergence is
> > achieved? Is it generally a good idea?
> >
> > Best regards,
> >
> > Michal Krompiec
> >
> > Associate Director at Merck KGaA and Adjunct Prof at University of
> > Southampton
> >
> >
> > On Sun, 19 May 2019 at 13:57, Nicola Marzari <nicola.marzari at epfl.ch
> > <mailto:nicola.marzari at epfl.ch>> wrote:
> >
> >
> >
> >     Hi Robert,
> >
> >     this issue of optimal parameters is a bit of a neverending story, we
> are
> >     preparing a short thing on this. But in a nutshell - yes, the
> >     electronic
> >     convergence should be tighter, especially for a follow-up phonon
> >     calculation. that number actually depends on the unit cell used, but
> >     for
> >     5 atoms, I would indeed try d-12 and d-14. For ecutwfc and ecutrho,
> >     well, my advise is to use the SSSP suggestions (google it); and say
> if
> >     you have 30 120, 40 160, 25 300, well, take the max of the ecutwfc
> >     suggested, for the gloval ecutwfc, and same for ecutrho.
> >
> >                              nic
> >
> >
> >     On 17/05/2019 21:24, Appleton, Robert J wrote:
> >      > Hello,
> >      >
> >      > I am trying to study the phonons of the material BaZrS3 to
> >     determine if
> >      > it is stable for a given structure. I first use phonopy and vasp
> and
> >      > found negative frequency values for phoning band structure. My
> >     advisor
> >      > is concerned in the optimization of the structure and so we now
> >     want to
> >      > use quantum espresso. I have only used it a few times and mostly
> >     used
> >      > default or example inputs. The input I want to make sure is
> >     sufficient
> >      > is my criteria for convergence.
> >      >
> >      > Under the &control:
> >      > force convergence criteria right now is
> >      > forc_conv_thr = .0001 <— this value seems to big but how do I
> >     choose a
> >      > value that is best for optimization but does not take to much
> >     computer time?
> >      >
> >      > Under the &electrons:
> >      > conv_thr = 1.d-9 <— I am considering changing to 1.d-12. Is this
> >     a good
> >      > idea?
> >      >
> >      > Under &system:
> >      > I am using ecutrho = 12 • ecutwfc
> >      >
> >      > If you can give me some advice on how best to choose convergence
> >      > criteria for studying phonons that would be very good thanks.
> >      >
> >      > Robert Appleton
> >      > Cal State LA - MS Physics Student
> >      >
> >      > Get Outlook for iOS <https://aka.ms/o0ukef>
> >      >
> >      > _______________________________________________
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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
> >     _______________________________________________
> >     Quantum ESPRESSO is supported by MaX
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> >     <http://www.max-centre.eu/quantum-espresso>)
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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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