<div><div dir="auto">Dear Nicola,</div></div><div dir="auto">Thank you very much!</div><div dir="auto">Regards,</div><div dir="auto">Michal </div><div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, 20 May 2019 at 20:04, Nicola Marzari <<a href="mailto:nicola.marzari@epfl.ch">nicola.marzari@epfl.ch</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><br>
<br>
Dear Michal,<br>
<br>
<br>
it depends on how smooth is the dependences of the forces on the cutoff,<br>
so one should make an informed decision depending on the <br>
pseudopotentials used. We e.g. look at phonon frequencies as a function <br>
of cutoff, and if you look at silicon, here below, even if we didn't <br>
test below 30Ry, I would bet that at 20 or even 15Ry forces would be <br>
reasonable:<br>
<br>
<a href="https://www.materialscloud.org/discover/data/discover/sssp/convergences_efficiency/Si_8.0_conv_patt.png" rel="noreferrer" target="_blank">https://www.materialscloud.org/discover/data/discover/sssp/convergences_efficiency/Si_8.0_conv_patt.png</a><br>
<br>
If you try Ga, you can see that the phonons (the solid line) do not<br>
particularly become worse in going down from 70Ry to 45Ry, but then <br>
progressively worsen:<br>
<br>
<a href="https://www.materialscloud.org/discover/data/discover/sssp/convergences_efficiency/Ga_8.0_conv_patt.png" rel="noreferrer" target="_blank">https://www.materialscloud.org/discover/data/discover/sssp/convergences_efficiency/Ga_8.0_conv_patt.png</a><br>
<br>
Still, for many pseudopotentials a decrease in cutoff doesn't worsen the <br>
forces particularly, but there is no 2/3 or 1/2 rule - one just need to<br>
examine the tables above.<br>
<br>
For ecutrho it might be a bit easier, and if you are not having <br>
magnetism often one can use a dual (the ration of ecutrho/ecutwfc)<br>
equalt to 3, or even 2, for normconserving, and 6 or even 4 for ultrasoft.<br>
<br>
All to be tested...<br>
<br>
nicola<br>
<br>
<br>
On 20/05/2019 13:47, Michal Krompiec wrote:<br>
> Dear Nicola,<br>
> I am looking forward to reading the "short thing on this"!<br>
> Can you comment on the widespread (?) practice of using much lower than <br>
> recommended (say, 2/3 or 1/2) ecutwfc for geometry optimization until a <br>
> minimum is found, and then increasing cutoffs until convergence is <br>
> achieved? Is it generally a good idea?<br>
> <br>
> Best regards,<br>
> <br>
> Michal Krompiec<br>
> <br>
> Associate Director at Merck KGaA and Adjunct Prof at University of <br>
> Southampton<br>
> <br>
> <br>
> On Sun, 19 May 2019 at 13:57, Nicola Marzari <<a href="mailto:nicola.marzari@epfl.ch" target="_blank">nicola.marzari@epfl.ch</a> <br>
> <mailto:<a href="mailto:nicola.marzari@epfl.ch" target="_blank">nicola.marzari@epfl.ch</a>>> wrote:<br>
> <br>
> <br>
> <br>
> Hi Robert,<br>
> <br>
> this issue of optimal parameters is a bit of a neverending story, we are<br>
> preparing a short thing on this. But in a nutshell - yes, the<br>
> electronic<br>
> convergence should be tighter, especially for a follow-up phonon<br>
> calculation. that number actually depends on the unit cell used, but<br>
> for<br>
> 5 atoms, I would indeed try d-12 and d-14. For ecutwfc and ecutrho,<br>
> well, my advise is to use the SSSP suggestions (google it); and say if<br>
> you have 30 120, 40 160, 25 300, well, take the max of the ecutwfc<br>
> suggested, for the gloval ecutwfc, and same for ecutrho.<br>
> <br>
> nic<br>
> <br>
> <br>
> On 17/05/2019 21:24, Appleton, Robert J wrote:<br>
> > Hello,<br>
> ><br>
> > I am trying to study the phonons of the material BaZrS3 to<br>
> determine if<br>
> > it is stable for a given structure. I first use phonopy and vasp and<br>
> > found negative frequency values for phoning band structure. My<br>
> advisor<br>
> > is concerned in the optimization of the structure and so we now<br>
> want to<br>
> > use quantum espresso. I have only used it a few times and mostly<br>
> used<br>
> > default or example inputs. The input I want to make sure is<br>
> sufficient<br>
> > is my criteria for convergence.<br>
> ><br>
> > Under the &control:<br>
> > force convergence criteria right now is<br>
> > forc_conv_thr = .0001 <— this value seems to big but how do I<br>
> choose a<br>
> > value that is best for optimization but does not take to much<br>
> computer time?<br>
> ><br>
> > Under the &electrons:<br>
> > conv_thr = 1.d-9 <— I am considering changing to 1.d-12. Is this<br>
> a good<br>
> > idea?<br>
> ><br>
> > Under &system:<br>
> > I am using ecutrho = 12 • ecutwfc<br>
> ><br>
> > If you can give me some advice on how best to choose convergence<br>
> > criteria for studying phonons that would be very good thanks.<br>
> ><br>
> > Robert Appleton<br>
> > Cal State LA - MS Physics Student<br>
> ><br>
> > Get Outlook for iOS <<a href="https://aka.ms/o0ukef" rel="noreferrer" target="_blank">https://aka.ms/o0ukef</a>><br>
> ><br>
> > _______________________________________________<br>
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> <br>
> <br>
> -- <br>
> ----------------------------------------------------------------------<br>
> Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL<br>
> Director, National Centre for Competence in Research NCCR MARVEL, EPFL<br>
> <a href="http://theossrv1.epfl.ch/Main/Contact" rel="noreferrer" target="_blank">http://theossrv1.epfl.ch/Main/Contact</a> <a href="http://nccr-marvel.ch/en/project" rel="noreferrer" target="_blank">http://nccr-marvel.ch/en/project</a><br>
> _______________________________________________<br>
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<br>
<br>
-- <br>
----------------------------------------------------------------------<br>
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL<br>
Director, National Centre for Competence in Research NCCR MARVEL, EPFL<br>
<a href="http://theossrv1.epfl.ch/Main/Contact" rel="noreferrer" target="_blank">http://theossrv1.epfl.ch/Main/Contact</a> <a href="http://nccr-marvel.ch/en/project" rel="noreferrer" target="_blank">http://nccr-marvel.ch/en/project</a><br>
</blockquote></div></div>