Dear Roberto, If I may jump in, I don't suppose it would be terribly costly to repeat one of your calculations with a cutoff that is slightly higher once you are done with the lower cutoff. You can compare whichever quantities you are interested in between the two cutoffs and decide whether the error is within a threshold that is self-determined/acceptable by the community at large. This would verify once and for all the appropriateness of your cutoff. Keep in mind however that for a system such as a graphene ribbon the cutoff may not be the only parameter to optimize. There's still the smearing and the amount of vaccum to worry about. For this reason, you should consider an isolated preliminary cutoff convergence study in a simpler system such as diamond or better still graphene as suggested in previous replies to your post.
Best wishes, Hande On Sun, 11 May 2008, Roberto G. A. Veiga wrote: > Hi, Stefano: > > as far as I understand, if I use a cutoff for a system made of carbons and I > find it is appropriate, I should use the same cutoff for another system also > made of carbons. I mean, if I use 20 Ry for carbon nanotubes and the results > are fine, I could use 20 Ry for diamond, or graphene. Is it wrong my > assumption? > > []s, > > Roberto > > Stefano Baroni <baroni at sissa.it> wrote: The kinetic-energy cutoff is a > property of the atomic core, NOT of the specific system (to a large extent).SB > > On May 10, 2008, at 6:11 PM, Roberto G. A. Veiga wrote: > > Dear Stefano: > > I'm using Ultrasoft PPs. My question was because this cutoff (20 Ry) seemed > to be appropriate for other carbonic systems. > > []s, > > Roberto > > Stefano Baroni <baroni at sissa.it> wrote: > On May 9, 2008, at 10:42 PM, Roberto G. A. Veiga wrote: > > Hi, Konstantin: > > thank you very much. Such a large cutoff (80 Ry) is necessary for > convergence? Or maybe I'm using one quite small (20 Ry)... > > it depends on the PP you are using, I guess, and certainly NOT on the actual > structure you are studying. 80 Ry seems like an appropriate cutoff for > moderately hard norm-conserving PP's, whereas 20 Ry sounds like appriate for > many US PP's. In either case, you better check the aprporiateness by > yourself, through thorough convergence tests. > > > Stefano > > > > > > []s, > > Roberto > > Konstantin Kudin <konstantin_kudin at yahoo.com> wrote: Hi Roberto, > > Here is a ribbon input that works for sure: > https://kiev.princeton.edu/misc/rg_07.scf.in > (get PPs from the PWSCF website) > > Good luck! > Kostya > --- > Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste > [+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 > > > > > > > > > > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum > > > > > --------------------------------- > Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it > now._______________________________________________ > 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 > [+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 > > > > > > > > > > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum > > > > --------------------------------- > Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it > now. -- Hande Ustunel Department of Physics Office 439 Middle East Technical University Ankara 06531, Turkey Tel : +90 312 210 3264 http://www.physics.metu.edu.tr/~hande
