Dear Ben,<div><br></div><div>I would also like to contribute my 5 cents to the discussion.</div><div>There are two main questions to answer prior to any convergence study.</div><div><br></div><div>1) What quantity is of interest?</div>
<div><br></div><div>If you can answer this question, you can save a lot of time and computational resources, because different</div><div>quantities require different parameters (cut-offs, BZ-sampling, etc.) to converge. Converging total energies</div>
<div>is not always a good idea.</div><div><br></div><div>2) How much precision do I really need?</div><div><br></div><div>Answering this question depends on the problem you are trying to solve. For instance, if you are trying to</div>
<div>identify which of two phases of some substance is most stable at 0K (according to DFT), and the difference</div><div>in the free energies of these phases is big, then you would need rather coarse parameters to figure out the answer</div>
<div>to your problem. It might happen though, that the difference is tiny, then you would have to tune the parameters</div><div>till a reliable distinction is achieved.</div><div><br></div><div>Best regards,</div><div>Maxim.</div>
<div><br></div><div><br><div class="gmail_quote">2013/2/25 Ali KACHMAR <span dir="ltr"><<a href="mailto:kachmar_ali@hotmail.fr" target="_blank">kachmar_ali@hotmail.fr</a>></span><br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div><div dir="ltr">
Dear Stefano,<br><br>That's great. In any case, I learnt too much from you and from many others on this forum.<br><br>Thank you,<br>Ali<br><br><div><div></div>> Date: Mon, 25 Feb 2013 15:05:14 +0100<br>> From: <a href="mailto:degironc@sissa.it" target="_blank">degironc@sissa.it</a><br>
> To: <a href="mailto:pw_forum@pwscf.org" target="_blank">pw_forum@pwscf.org</a><br>> Subject: Re: [Pw_forum] Technique for converging Ecut and K-points?<div><div class="h5"><br>> <br>> Dear All,<br>> My previous post was actually more intended as an answer to Ben <br>
> Palmer question than a comment to<br>> Ali Kachmar contribution. Sorry.<br>> best regards,<br>> stefano<br>> <br>> <br>> On 02/25/2013 02:58 PM, Stefano de Gironcoli wrote:<br>> > Dear Ali Kachmar,<br>
> ><br>> > convergence w.r.t. ecutwfc (and ecutrho) and convergence w.r.t. <br>> > k-points sampling are rather independent issues and can be tested to a <br>> > large extent separately<br>> ><br>
> > - convergence w.r.t. ecutwfc and ecutrho is a property depending on <br>> > the highest Fourier components that are needed to describe the <br>> > wavefunctions and the density of your system. his depends on the <br>
> > pseudopotentials that are present in the calculation and do not depend <br>> > strongly, for a given set of pseudopotentials, on the particular <br>> > configuration because it depends mostly on the behaviour of the wfc in <br>
> > the core region which is quite insensitive (in terms of shape) on the <br>> > environment.<br>> > So each pseudopotential has a required cutoff. An upperbound to this <br>> > value can be determined from any system that contains that pseudo.<br>
> > The cutoff needed for a system containing several species is the <br>> > highest among those needed for each element.<br>> > Moreover, in US pseudo or PAW the charge density has contributions <br>> > from localized terms that may (an usually do in USPP) require quite <br>
> > higher cutoff than the one needed for psi**2 (4*ecutwfc) ... hence the <br>> > possibility to vary and test independently for ecutrho ...<br>> ><br>> > My recommended strategy to fix ecutwfc and ecutrho is to perform total <br>
> > energy (and possibly, force and stress) covergence test increasing <br>> > ecutwfc keeping ecutrho at its default vaule (=4*ecutwfc) until <br>> > satisfactory stability is reached (typically ~1 mry/atom in the <br>
> > energy, 1.d-4 ry/au in the forces, a fraction of a KBar in the stress) <br>> > ... this fixes the converged value of ecutrho to 4 times the <br>> > resulting ecutwfc.<br>> > Now keeping this value for ecutrho one can try to reduce ecutwfc and <br>
> > see how much this can be done without deteriorating the convergence.<br>> ><br>> > -convergence with respect to k-points is a property of the band <br>> > structure.<br>> > I would study it after the ecutwfc/ecutrho issue is settled but some <br>
> > fairly accurate parameters can be obtained even with reasonable but <br>> > not optimal cutoff parameters.<br>> ><br>> > There is a big difference between convergence in a band insulator or <br>
> > in a metal.<br>> ><br>> > In an insulator bands are completely occupied or empty across the BZ <br>> > and charge density can be written in terms of wannier functions that <br>> > are exponentially localized in real space.<br>
> > Hence the convergence w.r.t the density of point in the different <br>> > directions in the BZ should be exponentially fast and anyway quite <br>> > quick...<br>> ><br>> > In a metal the need to sample only a portion of the BZ would require <br>
> > an extremely dense set of k points in order to locate accurately the <br>> > Fermi surface. This induces to introduce a smearing width that smooth <br>> > the integral to be performed... the larger the smearing width, the <br>
> > smoother the function, and the faster the convergence results...<br>> > however the larger the smearing width the farther the result is going <br>> > to be from the accurate, zero smearing width, result that one would <br>
> > desire.<br>> > Therefore different shapes fro the smearing functions have been <br>> > proposed to alleviate this problem and<br>> > Marzari-Vanderbilt and Methfessel-Paxton smearing functions give a <br>
> > quite mild dependence of the (k-point converged) total energy as a <br>> > function of the smearing width thus being good choices for metals.<br>> ><br>> > My recommended strategy for fix the k-point sampling in metals is<br>
> > 1) chose the smearing function type (mv or mp, recomended)<br>> > 2) for decreasing values of the smearing width (let's say from an high <br>> > value of 0.1 ry = 1.36 eV to a low value of 0.01 - 0.005 ry = <br>
> > 0.136-0.068 eV if feasable) CONVERGE the total energy w.r.t to <br>> > smearing well within the global desired tolerance (of 1 mry/atom, for <br>> > instance)<br>> > 3) by examining the behaviour of the CONVERGED Energy vs smearing <br>
> > width curve E(sigma) identify the smearing width for which E(sigma) is <br>> > within tolerance w.r.t. E(sigma==0) keeping in mind that for <br>> > methfessel-paxton E(sigma) ~ E(0) + A*sigma**4 + o(sigma**6) while for <br>
> > marzari-vanderbilt the dependence is more likely E(sigma) ~ E(0) <br>> > +A*sigma**3 + o(sigma**4).<br>> > 4) select that value of the smearing width and the smallest set of <br>> > k-points for which this is converged.<br>
> ><br>> > HTH<br>> ><br>> > stefano<br>> ><br>> ><br>> ><br>> > On 02/24/2013 06:54 PM, Ali KACHMAR wrote:<br>> >> Hi,<br>> >><br>> >> as far as I know, there is no any techinques for choosing ecut and <br>
> >> k-points. Please have a look at the pwscf archive and make up a <br>> >> conclusion.<br>> >><br>> >> Best,<br>> >> Ali<br>> >><br>> >>> Date: Sat, 23 Feb 2013 19:55:51 +0000<br>
> >>> <a href="mailto:From%3Abenpalmer1983@gmail.com" target="_blank">From:benpalmer1983@gmail.com</a><br>> >>> <a href="mailto:To%3Apw_forum@pwscf.org" target="_blank">To:pw_forum@pwscf.org</a><br>
> >>> Subject: [Pw_forum] Technique for converging Ecut and K-points?<br>> >>><br>> >>> Hi everyone,<br>> >>><br>> >>> I just wanted to ask if users have any techniques for choosing ecut and<br>
> >>> k-points? I've read that one way would be to start with a high number<br>> >>> of k-points and high energy cutoff, and use that energy as an almost<br>> >>> true value. Then adjust k-points and energy cutoff from a lower<br>
> >>> number/cutoff until it converges to the true value. Would you try to<br>> >>> converge energy cutoff first, or k-points? Does it matter which you<br>> >>> converge first?<br>> >>><br>
> >>> Thanks<br>> >>><br>> >>> Ben Palmer<br>> >>> Student @ University of Birmingham<br>> >>> _______________________________________________<br>> >>> Pw_forum mailing list<br>
> >>> <a href="mailto:Pw_forum@pwscf.org" target="_blank">Pw_forum@pwscf.org</a><br>> >>> <a href="http://pwscf.org/mailman/listinfo/pw_forum" target="_blank">http://pwscf.org/mailman/listinfo/pw_forum</a><br>
> >><br>> >><br>> >><br>> >> _______________________________________________<br>> >> Pw_forum mailing list<br>> >> <a href="mailto:Pw_forum@pwscf.org" target="_blank">Pw_forum@pwscf.org</a><br>
> >> <a href="http://pwscf.org/mailman/listinfo/pw_forum" target="_blank">http://pwscf.org/mailman/listinfo/pw_forum</a><br>> ><br>> ><br>> <br>> _______________________________________________<br>
> Pw_forum mailing list<br>> <a href="mailto:Pw_forum@pwscf.org" target="_blank">Pw_forum@pwscf.org</a><br>> <a href="http://pwscf.org/mailman/listinfo/pw_forum" target="_blank">http://pwscf.org/mailman/listinfo/pw_forum</a><br>
</div></div></div> </div></div>
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<a href="http://pwscf.org/mailman/listinfo/pw_forum" target="_blank">http://pwscf.org/mailman/listinfo/pw_forum</a><br></blockquote></div><br><br clear="all"><div><br></div>-- <br>Best regards, Max Popov<br>Ph.D. student<br>
Materials center Leoben (MCL), Leoben, Austria.
</div>