Dear Stefano, That's great. In any case, I learnt too much from you and from many others on this forum.
Thank you, Ali > Date: Mon, 25 Feb 2013 15:05:14 +0100 > From: degironc at sissa.it > To: pw_forum at pwscf.org > Subject: Re: [Pw_forum] Technique for converging Ecut and K-points? > > Dear All, > My previous post was actually more intended as an answer to Ben > Palmer question than a comment to > Ali Kachmar contribution. Sorry. > best regards, > stefano > > > On 02/25/2013 02:58 PM, Stefano de Gironcoli wrote: > > Dear Ali Kachmar, > > > > convergence w.r.t. ecutwfc (and ecutrho) and convergence w.r.t. > > k-points sampling are rather independent issues and can be tested to a > > large extent separately > > > > - convergence w.r.t. ecutwfc and ecutrho is a property depending on > > the highest Fourier components that are needed to describe the > > wavefunctions and the density of your system. his depends on the > > pseudopotentials that are present in the calculation and do not depend > > strongly, for a given set of pseudopotentials, on the particular > > configuration because it depends mostly on the behaviour of the wfc in > > the core region which is quite insensitive (in terms of shape) on the > > environment. > > So each pseudopotential has a required cutoff. An upperbound to this > > value can be determined from any system that contains that pseudo. > > The cutoff needed for a system containing several species is the > > highest among those needed for each element. > > Moreover, in US pseudo or PAW the charge density has contributions > > from localized terms that may (an usually do in USPP) require quite > > higher cutoff than the one needed for psi**2 (4*ecutwfc) ... hence the > > possibility to vary and test independently for ecutrho ... > > > > My recommended strategy to fix ecutwfc and ecutrho is to perform total > > energy (and possibly, force and stress) covergence test increasing > > ecutwfc keeping ecutrho at its default vaule (=4*ecutwfc) until > > satisfactory stability is reached (typically ~1 mry/atom in the > > energy, 1.d-4 ry/au in the forces, a fraction of a KBar in the stress) > > ... this fixes the converged value of ecutrho to 4 times the > > resulting ecutwfc. > > Now keeping this value for ecutrho one can try to reduce ecutwfc and > > see how much this can be done without deteriorating the convergence. > > > > -convergence with respect to k-points is a property of the band > > structure. > > I would study it after the ecutwfc/ecutrho issue is settled but some > > fairly accurate parameters can be obtained even with reasonable but > > not optimal cutoff parameters. > > > > There is a big difference between convergence in a band insulator or > > in a metal. > > > > In an insulator bands are completely occupied or empty across the BZ > > and charge density can be written in terms of wannier functions that > > are exponentially localized in real space. > > Hence the convergence w.r.t the density of point in the different > > directions in the BZ should be exponentially fast and anyway quite > > quick... > > > > In a metal the need to sample only a portion of the BZ would require > > an extremely dense set of k points in order to locate accurately the > > Fermi surface. This induces to introduce a smearing width that smooth > > the integral to be performed... the larger the smearing width, the > > smoother the function, and the faster the convergence results... > > however the larger the smearing width the farther the result is going > > to be from the accurate, zero smearing width, result that one would > > desire. > > Therefore different shapes fro the smearing functions have been > > proposed to alleviate this problem and > > Marzari-Vanderbilt and Methfessel-Paxton smearing functions give a > > quite mild dependence of the (k-point converged) total energy as a > > function of the smearing width thus being good choices for metals. > > > > My recommended strategy for fix the k-point sampling in metals is > > 1) chose the smearing function type (mv or mp, recomended) > > 2) for decreasing values of the smearing width (let's say from an high > > value of 0.1 ry = 1.36 eV to a low value of 0.01 - 0.005 ry = > > 0.136-0.068 eV if feasable) CONVERGE the total energy w.r.t to > > smearing well within the global desired tolerance (of 1 mry/atom, for > > instance) > > 3) by examining the behaviour of the CONVERGED Energy vs smearing > > width curve E(sigma) identify the smearing width for which E(sigma) is > > within tolerance w.r.t. E(sigma==0) keeping in mind that for > > methfessel-paxton E(sigma) ~ E(0) + A*sigma**4 + o(sigma**6) while for > > marzari-vanderbilt the dependence is more likely E(sigma) ~ E(0) > > +A*sigma**3 + o(sigma**4). > > 4) select that value of the smearing width and the smallest set of > > k-points for which this is converged. > > > > HTH > > > > stefano > > > > > > > > On 02/24/2013 06:54 PM, Ali KACHMAR wrote: > >> Hi, > >> > >> as far as I know, there is no any techinques for choosing ecut and > >> k-points. Please have a look at the pwscf archive and make up a > >> conclusion. > >> > >> Best, > >> Ali > >> > >>> Date: Sat, 23 Feb 2013 19:55:51 +0000 > >>> From:benpalmer1983 at gmail.com > >>> To:pw_forum at pwscf.org > >>> Subject: [Pw_forum] Technique for converging Ecut and K-points? > >>> > >>> Hi everyone, > >>> > >>> I just wanted to ask if users have any techniques for choosing ecut and > >>> k-points? I've read that one way would be to start with a high number > >>> of k-points and high energy cutoff, and use that energy as an almost > >>> true value. Then adjust k-points and energy cutoff from a lower > >>> number/cutoff until it converges to the true value. Would you try to > >>> converge energy cutoff first, or k-points? Does it matter which you > >>> converge first? > >>> > >>> Thanks > >>> > >>> Ben Palmer > >>> Student @ University of Birmingham > >>> _______________________________________________ > >>> Pw_forum mailing list > >>> Pw_forum at pwscf.org > >>> http://pwscf.org/mailman/listinfo/pw_forum > >> > >> > >> > >> _______________________________________________ > >> Pw_forum mailing list > >> Pw_forum at pwscf.org > >> http://pwscf.org/mailman/listinfo/pw_forum > > > > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20130225/3e93de22/attachment.html
