Il giorno 22/nov/2010, alle ore 19.18, Dhruv Singh ha scritto: > Thanks very much for the help regarding this topic.... I guess the > hexagonal supercell is a better choice then and I will need to use a
If there's no specific reason for using a lower symmetry cell (as the tetragonal one you've been using previously), I think so. > finer k point grid with tighter scf convergence criterion for better > accuracy in forces. As an advice though, would you suggest that for > subsequent force constant calculations - forces of the order of 1.0e-5 That's a very tight threshold! i.e. 2 orders of magnitudes smaller than the default one... > would not lead to unstable modes ? (I am using a supercell to > calculate force constants rather than the ph.x codes because I later > need to calculate third derivatives) > > As regards the 72 atom supercell scf calculations not converging, the > corresponding input file is attached here. I increased the scf > convergence threshold to 1.0e-3 in order to terminate the computation > earlier (to check the results). I am attaching two output files-one > with threshold 1.0e-8 and the other with 1.0e-3. Both of them use a 72 > atom supercell and a 2x2x1 kpoint grid. Please let me know if you > think tweaking a few parameters could help convergence. Graphene has zero bandgap, hence you might need to include a broadening of the electronic occupations (i.e. you need to specify also occupations and smearing keywords, degauss is not sufficient). HTH GS > > Thanks very much > > Dhruv > > On Mon, Nov 22, 2010 at 8:26 AM, Paolo Giannozzi <giannozz at democritos.it> > wrote: >> >> On Nov 22, 2010, at 14:23 , Gabriele Sclauzero wrote: >> >>> Hence, if you want more precision you probably need to use >>> better k-point sampling and C-C distances. >> >> and better (i.e. smaller) self-consistenty convergence thresholds. >> The error on the energy is a quadratic function of the error on the >> charge density due to imperfect self-consistency; the error on forces >> is instead linear. >> >> P. >> --- >> Paolo Giannozzi, Dept of Chemistry&Physics, Univ. Udine >> via delle Scienze 208, 33100 Udine, Italy >> Phone +39-0432-558216, fax +39-0432-558222 >> >> >> >> >> _______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org >> http://www.democritos.it/mailman/listinfo/pw_forum >> > > > > -- > Dhruv > <gra.scf.in><gra2by2_thr1.0e-3.scf.out><gra2by2_thr1e-8.scf.out>_______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum ? Gabriele Sclauzero, EPFL SB ITP CSEA PH H2 462, Station 3, CH-1015 Lausanne -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20101123/0a4f5241/attachment.htm
