I would be surprised if roundoff errors in FFT should be blamed for the reported behavior SB
On Jan 25, 2010, at 11:34 AM, O. Baris Malcioglu wrote: > Dear Brad, > > Apart from what is already mentioned, another contribution to error > you are mentioning might be due to computational/numeric issues. > > Run-of-the-mill FFT algorithms has an error upperbound that goes like > O(sqrt(N)). Even in the best case (least performing) scenarios the > error upper bound is O(log(N)). > > These errors are due to numeric precision employed, and mostly bound > by hardware (unless you compromise performance significantly). > > In general, in any calculation that involves computers, in a graph > that shows the "error" as a function of some convergent parameter, I > would expect three regions , first the error diminishes due to > numerical reasons, then there is a flat region, then roundoff errors > begin to dominate, and error picks up again. > > If interested, please have a look at the (rather rusty but still > valid) concept "Machine constant" > > Best, > Baris. > > > > 2010/1/25 Paolo Giannozzi <giannozz at democritos.it>: >> Brad Malone wrote: >>> I originally saw this problem with espresso-4.1.1 in a >>> different system (AlAs on a 2x2x2 shifted grid). >> >> start with 'random' or 'atomic+random' initial wavefunctions >> (startingwfc='...'). I didn't find anything wrong with very >> high cutoffs. Occasionally you can end up in the wrong ground >> state, though, especially in highly symmetric cases like the >> Si example you sent. >> >> "Ghost states" are another story. It may happen that a >> (nonlocal, separable) pseudopotential has a localized >> spurious state that doesn't show up (i.e. it is not >> occupied) at low cutoff, but it is occupied at higher >> cutoff. None of the PP on the QE web site should have >> such states, though. >> >> If you have evidence of further problems, please provide >> a test case so that it can be reproduced. In any case, >> thank you for finding a bug in the cvs version! >> >> P. >> --- >> Paolo Giannozzi, Democritos and University of Udine, Italy >> _______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org >> http://www.democritos.it/mailman/listinfo/pw_forum >> > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum --- Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype) La morale est une logique de l'action comme la logique est une morale de la pens?e - Jean Piaget Please, if possible, don't send me MS Word or PowerPoint attachments Why? See: http://www.gnu.org/philosophy/no-word-attachments.html -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20100125/a1a0b8ae/attachment.htm
