Dear Stefano, I've just finished running the calculations, and I've quickly put the results into excel. K-points were 6,8,10,12,14,16,18.
Energy v K-points: The horizontal lines are +/- 1mRy from the energy with 18x18x18 k-points. PWscf runtime v K-points: Would it be reasonable to choose k-points 10, maybe even 8, and any smearing width? I could try higher k-points, but it may take a while as my computer is a bit slow. All the best, Ben On 27/02/2013 10:40, Stefano de Gironcoli wrote: > Dear Ben, > > 2x2x2 k-points with a 0.02 Ry width looks to me quite a daring small > k-points set ... I would start with a somewhat denser 6x6x6 or 8x8x8 one... > > the first check of convergence with respect to ecutwfc/ecutrho is > fine BUT your interpretation is not. > what you conclude from there is that ecutwf=35 and ecutrho=140 are > enough to converge the energy > in particular ecutrho=140 is enough ! > > The second stage is therefore to fix ecutrho=140 and check whether > it is possible to REDUCE ecutwfc. > > As for the convergence w.r.t. to k-points i would make a more > systematic study... > reordering the data that you present... and examining results for > decreasing values of the smearing width > > width = 0.03 > 9956.6765: K-point: 6, Degauss: 0.03, energy: -154.05968822, time: > 0m46.50s > 9956.6735: K-point: 8, Degauss: 0.03, energy: -154.06335682, time: > 1m20.19s ? > there is not enough information to decide what grid would be enough here... > definitely you should test for 12, 16 (23 if needed) etc > > width = 0.02 > 9956.6754: K-point: 6, Degauss: 0.02, energy: -154.05986685, time: > 0m46.42s > 9956.6720: K-point: 8, Degauss: 0.02, energy: -154.06351667, time: 1m > 8.18s > 9956.6473: K-point: 12, Degauss: 0.02, energy: -154.06227228, time: > 3m59.45s * > 9956.6243: K-point: 16, Degauss: 0.02, energy: -154.0622109, time: > 6m41.03s > a nicely complete series of calculations > 12x12x12 is converged within a fraction of mry ... > > width = 0.015 > 9956.6191: K-point: 16, Degauss: 0.015, energy: -154.06218801, time: > 6m40.66s * > 9956.6122: K-point: 24, Degauss: 0.015, energy: -154.06234325, time: > 27m28.22s > 16x16x16 is converged withinn a fraction of mry... maybe 12 (or less) is > enough but you didn't check > > width = 0.01 > 9956.6358: K-point: 12, Degauss: 0.01, energy: -154.06191016, time: > 3m47.69s > 9956.6064: K-point: 24, Degauss: 0.01, energy: -154.06231018, time: > 27m31.26s ?* > who knows if (as probable) 24x24x24 is converged ...or even > overkilling... missing data at 16 and 20 would help to understand > > width = 0.005 > 9956.6004: K-point: 24, Degauss: 0.005, energy: -154.06230709, time: > 25m21.81s ? > again... no clue whether this is converged or not... data at 12,16,20... > (and 28,32 if needed) are missing > > once converged results as function of smearing width are obtained > you can decide which is the largest width that is still accurate and > then what is the smallest k-point set that > integrates it correctly... > > width converged energy converged grid > 0.03 ? > 0.02 -154.06227228, * 12 > 0.015 -154.06234325, * 16 > 0.01 -154.06231018, ?* 24 ? > 0.005 ? > > looks like 0.02 and 12x12x12 is fine within a fraction of mry... > maybe also 0.03 or larger with a smaller grid but there is no info to tell. > > HTH > > stefano > >> 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 > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum -------------- next part -------------- An HTML attachment was scrubbed... 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