Dear Stefano and Paolo, I think that information from this thread should be placed into the User Manual and FAQ. :)
2013/2/27 Ben Palmer <benpalmer1983 at gmail.com> > Dear Stefano, > > I have had a go at converging for Aluminium. I wasn't too sure what to > do with the K-points, but I've had a go anyway. This is what I have > done, step by step, with some results from the calculations. > > First I set k-points to 2, smearing as MV and a width of 0.02 for the > energy convergence. I varied the energy from 10 to 50 (taking 50 as the > 'true' value), and selected the first within 1mRy of the 'true' energy. > > 113.2226: K points: 2, ecut: 10, energy: -153.984951, time: 4.05s > 113.2237: K points: 2, ecut: 15, energy: -153.99949084, time: 4.31s > 113.2251: K points: 2, ecut: 20, energy: -154.00496366, time: 4.96s > 113.2261: K points: 2, ecut: 25, energy: -154.00841338, time: 5.49s > 113.2268: K points: 2, ecut: 30, energy: -154.00937872, time: 6.52s > 113.2283: K points: 2, ecut: 35, energy: -154.01004897, time: 8.31s > 113.2292: K points: 2, ecut: 40, energy: -154.01057988, time: 8.17s > 113.2306: K points: 2, ecut: 45, energy: -154.01066204, time: 10.91s > 113.2314: K points: 2, ecut: 50, energy: -154.01083456, time: 12.83s > 113.2315: Converged energy cutoff ecutwfc: 35 > > I then lowered ecutrho until, and selected the lowest value that fell > within 1mRy of the 'true' energy. > > 218.6003: K points: 2, ecutwfc: 35, ecutrho: 252, energy: > -154.01105461, time: 10.98s > 218.6014: K points: 2, ecutwfc: 35, ecutrho: 224, energy: > -154.01104417, time: 11.67s > 218.6031: K points: 2, ecutwfc: 35, ecutrho: 196, energy: > -154.01069558, time: 8.82s > 218.604: K points: 2, ecutwfc: 35, ecutrho: 168, energy: > -154.01054112, time: 9.99s > 218.6052: K points: 2, ecutwfc: 35, ecutrho: 140, energy: > -154.01004897, time: 8.46s > 218.6064: K points: 2, ecutwfc: 35, ecutrho: 112, energy: > -154.00962358, time: 7.22s > 218.6076: K points: 2, ecutwfc: 35, ecutrho: 84, energy: > -154.00782952, time: 5.80s > 218.6082: Converged energy cutoff ecutrho: 140 > > At this point, I've got ecutwfc = 35 and ecutrho = 140, but I wasn't too > sure how to progress, so I attempted the following. I set a large > number of k-points, 24x24x24, with a narrow smearing of 0.005. I used > the energy cutoffs to then calculate a new reference energy for > convergence. > > I increased the smear width and decreased the k-points in quite > arbitrary combinations, and looked for the combination that executed > fastest, while keeping within 1mRy of the new reference energy. > > 9956.6004: K-point: 24, Degauss: 0.005, energy: -154.06230709, time: > 25m21.81s > 9956.6064: K-point: 24, Degauss: 0.01, energy: -154.06231018, time: > 27m31.26s > 9956.6122: K-point: 24, Degauss: 0.015, energy: -154.06234325, time: > 27m28.22s > 9956.6191: K-point: 16, Degauss: 0.015, energy: -154.06218801, time: > 6m40.66s > 9956.6243: K-point: 16, Degauss: 0.02, energy: -154.0622109, time: > 6m41.03s > 9956.6358: K-point: 12, Degauss: 0.01, energy: -154.06191016, time: > 3m47.69s > 9956.6473: K-point: 12, Degauss: 0.02, energy: -154.06227228, time: > 3m59.45s > 9956.672: K-point: 8, Degauss: 0.02, energy: -154.06351667, time: 1m > 8.18s > 9956.6735: K-point: 8, Degauss: 0.03, energy: -154.06335682, time: > 1m20.19s > 9956.6754: K-point: 6, Degauss: 0.02, energy: -154.05986685, time: > 0m46.42s > 9956.6765: K-point: 6, Degauss: 0.03, energy: -154.05968822, time: > 0m46.50s > > From this, I'd choose K-points 12x12x12 and smearing width 0.01 or 0.02. > > My final convergence settings were: > > ecutwfc = 35, > ecutrho = 140, > k points 12x12x12 > smearing mv 0.01 > > Would this be an acceptable way to chose the settings, or could I speed > up the end part? > > All the best, > > Ben Palmer, Student @ University of Birmingham > > > 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 > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum > -- ? ?????????, ??????? ??????? Best regards, Dr. Dmitry Korotin Institute of Metal Physics S. Kovalevskaya, 18 620990 Yekaterinburg Russia -------------- next part -------------- An HTML attachment was scrubbed... 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