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 -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20130225/4ddea6eb/attachment.html
