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 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 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. 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 -------------- next part -------------- A non-text attachment was scrubbed... Name: gra.scf.in Type: application/octet-stream Size: 4820 bytes Desc: not available Url : http://www.democritos.it/pipermail/pw_forum/attachments/20101122/c154e516/attachment-0003.obj -------------- next part -------------- A non-text attachment was scrubbed... Name: gra2by2_thr1.0e-3.scf.out Type: application/octet-stream Size: 32857 bytes Desc: not available Url : http://www.democritos.it/pipermail/pw_forum/attachments/20101122/c154e516/attachment-0004.obj -------------- next part -------------- A non-text attachment was scrubbed... Name: gra2by2_thr1e-8.scf.out Type: application/octet-stream Size: 28263 bytes Desc: not available Url : http://www.democritos.it/pipermail/pw_forum/attachments/20101122/c154e516/attachment-0005.obj
