And as expected, the termination threshold of 1.0e-3 gives a rather lousy answer for forces and energy/atom of -11.39486969 Ry significantly different from the correct value of ~ -11.395192 Ry. I believe a finer kpoint grid should resolve the convergence issue to a certain extent (correct me if I am wrong) - but I am trying to understand if a well converged asnwer can be obtained even on a 2x2x1 kpoint grid.
Thanks, Dhruv On Mon, Nov 22, 2010 at 1:18 PM, Dhruv Singh <dhruv.singh at gmail.com> wrote: > 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 > -- Dhruv
