[Pw_forum] vc-relax

[hania djani-ait aissa]

Dear Tim,here attached my grep on volume.
cheers
Hania Djani ait aissa
Algeria> Date: Sat, 24 Jan 2009 10:36:57 -0800> From: thmason24 at yahoo.com> To: pw_forum at pwscf.org> CC: akohlmey at cmm.chem.upenn.edu> Subject: Re: [Pw_forum] vc-relax> > Axel,> > thanks for the insight. That is outstanding information. I expect many people are running into this problem (I've seen the force oscillation energy decrease phenomenom posted many times). I am using what I thought were conservative ecuts (40 ryd) but perhaps not big enough for the stress tensor when using USPP. This characteristic would be a great thing to warn people about in the wiki vc-relax section. I'd offer to do it myself but I'd feel better leaving it to an expert. I'll report back if this improves my calculation but with the higher ecuts it won't be for a while. Is the issue confined to USPP? would you recommend NCPP for VC-relax as a work around?> > > > > Tim Mason> University of Missouri St. Louis> > > > > > > On Sat, Jan 24, 2009 at 12:12 PM, Axel Kohlmeyer <akohlmey at cmm.chem.upenn.edu> wrote:> > On Sat, 24 Jan 2009, Timothy Mason wrote:> > TM> Dear Hania,> TM>> > TM> as someone else struggling with VC-relax, I'd like to see a grep of> TM> your cell volumes. (just grep on volume). I find that in these> TM> cases where the forces oscillate but the engergies decrease (all> TM> cases for me) the volumes continue to expand about linearly with> TM> time. I'm not sure what to make of it. It even happens starting> > good point. that could very likely mean, that your stress> tensor is not sufficiently converged wrt. to the wavefunction> or density cutoff. one of the most problematic issues with> vc-relax calculations is that the stress tensor need a much> higher wavefunction cutoff (and very much highe density cutoff> in case of ultra-soft pseudos) to be well converged. to make> matters worse, in a vc-relax calculations, you do not keep> those cutoffs constant, but the number of plane waves. so for> an expanding cell you effectively decrease the cutoffs, and> if you stress tensor is not well converged, it will become> quickly not converged. there are some tricks to make this> effect less of a problem, but the immediate solution is to> start at a lattice constant that is definitely too large,> i.e. the cell will never expand beyond it. you still have> consider pulay stress, but that is relatively minor concern.> > TM> with experimental structures often increasing them by 50 percent> TM> before convergence thresholds are met.> > a 50% increase sounds unphysical. have you tried restarting> from the final configuration (resets the "effective" basis> set size) and see if the results are consistent?> > cheers,> axel.> > > _______________________________________________> Pw_forum mailing list> Pw_forum at pwscf.org> http://www.democritos.it/mailman/listinfo/pw_forum
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