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.
