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