Axel, Your advices are valuable, thanks a lot!. Seeing most of my calculations (geometry optimization) are still effective, I can set my heart at rest :-).
Dr. Huiqun Zhou @Nanjing University ----- Original Message ----- From: "Axel Kohlmeyer" <[email protected]> To: "Huiqun Zhou" <hqzhou at nju.edu.cn> Cc: <pw_forum at pwscf.org> Sent: Wednesday, November 14, 2007 1:31 PM Subject: Re: [Pw_forum] Convergence test > On Tue, 13 Nov 2007, Huiqun Zhou wrote: > > HZ> List users, > > dear huiqun zhou, > > HZ> I'm testing convergence on ecutwfc for chromite using USPP. Below are > HZ> results of different ecutrho with different ecutwfc. I may be wrong, I > think > HZ> I should watch the difference of total energy between successive runs > till > HZ> it's less than 1-2 mRy. But the trend is not apparent in the results > below > HZ> and it seems that the larger the ecutwfc, the lower the total energy. > > you are using an ultra-soft pseudopotential and for those > actually the density cutoff is _much_ more important than > the wavefunction cutoff. depending on the choice of the > pseudopotential parameters, 35-40ry for the wavefunction > cutoff should be a conservative choice, however a density > cutoff of 250 is not. if you want to converge the energy > very tightly, you may need to go up to 10x the wavefunction > cutoff. you should also keep in mind that the total energy > converges rather slowly, and that for most calculations > you need energy differences or derivatives, which may converge > faster, i.e. you may want to look at forces (use a non-symmetric > input for that!!) or stress instead. what is sufficiently > converged depends on your the kind of calculation you want > to do. so for simple geometry optimization or MD 35/210 > or less may already be sufficient. > > HZ> > HZ> I calculated E-V relations using two sets of ecutwfc and ecutrho: > HZ> for ecutwfc = 35 ry and ecutrho = 240 ry, a = 8.405, B0 = 172.8, B0' = > 4.3 > HZ> for ecutwfc = 55 ry and ecutrho = 250 ry, a = 8.407, B0 = 178.6, B0' = > 3.6 > HZ> > HZ> There are no big differences in unit cell parameter and elastic > properties. > HZ> So, my questions are > HZ> (1) usually, the ecutwfc of USPP should be around 35 ry or below, is > my > HZ> case an exception? > > no. it does not look like an exception. > > HZ> (2) how small the difference of total energy of successive runs should > be > HZ> in judging convergence against ecutwfc? > > total energy is not a good parameter. see above. > > cheers, > axel. > > HZ> > HZ> Thank you in advance. > HZ> > HZ> Dr. Huiqun Zhou > HZ> @Earth Sciences, Nanjing University, China > HZ> > HZ> > HZ> > HZ> > HZ> ecutrho = 240 ry > HZ> ecutwfc total energy > HZ> 35 -1461.54573299 Ry > HZ> 40 -1461.64206824 Ry > HZ> 45 -1461.71978835 Ry > HZ> 50 -1461.76587314 Ry > HZ> 55 -1461.76316597 Ry > HZ> 60 -1461.76692396 Ry > HZ> > HZ> ecutrho = 250 ry > HZ> 40 -1461.64351876 Ry > HZ> 45 -1461.70130510 Ry > HZ> 50 -1461.74724764 Ry > HZ> 55 -1461.76462943 Ry > HZ> 60 -1461.76838812 Ry > HZ> > HZ> ecutrho = 280 ry > HZ> 50 -1461.74778158 Ry > HZ> 55 -1461.76516515 Ry > HZ> 60 -1461.76892327 Ry > HZ> > HZ> _______________________________________________ > HZ> Pw_forum mailing list > HZ> Pw_forum at pwscf.org > HZ> http://www.democritos.it/mailman/listinfo/pw_forum > HZ> > > -- > ======================================================================= > Axel Kohlmeyer akohlmey at cmm.chem.upenn.edu http://www.cmm.upenn.edu > Center for Molecular Modeling -- University of Pennsylvania > Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323 > tel: 1-215-898-1582, fax: 1-215-573-6233, office-tel: 1-215-898-5425 > ======================================================================= > If you make something idiot-proof, the universe creates a better idiot. >
