<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>Il giorno 22/nov/2010, alle ore 19.23, Dhruv Singh ha scritto:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div>And as expected, the termination threshold of 1.0e-3 gives a rather<br>lousy answer for forces and energy/atom of -11.39486969 Ry<br>significantly different from the correct value of ~ -11.395192 Ry. I<br></div></blockquote><div><br></div><div>I wouldn't say "lousy"... you got what you asked for, that is to a say an accuracy of 1e-3 Ry on the total energy.</div><br><blockquote type="cite"><div>believe a finer kpoint grid should resolve the convergence issue to a<br>certain extent (correct me if I am wrong) - but I am trying to<br>understand if a well converged asnwer can be obtained even on a 2x2x1<br>kpoint grid.</div></blockquote><div><br></div><div>if you use a n x n supercell, you should get the same accuracy of the primitive unit cell by reducing the k-point mesh accordingly (e.g. k_x/n k_y/n)</div><div><br></div><div>GS</div><br><blockquote type="cite"><div><br>Thanks,<br>Dhruv<br><br>On Mon, Nov 22, 2010 at 1:18 PM, Dhruv Singh <<a href="mailto:dhruv.singh@gmail.com">dhruv.singh@gmail.com</a>> wrote:<br><blockquote type="cite">Thanks very much for the help regarding this topic.... I guess the<br></blockquote><blockquote type="cite">hexagonal supercell is a better choice then and I will need to use a<br></blockquote><blockquote type="cite">finer k point grid with tighter scf convergence criterion for better<br></blockquote><blockquote type="cite">accuracy in forces. As an advice though, would you suggest that for<br></blockquote><blockquote type="cite">subsequent force constant calculations - forces of the order of 1.0e-5<br></blockquote><blockquote type="cite">would not lead to unstable modes ? (I am using a supercell to<br></blockquote><blockquote type="cite">calculate force constants rather than the ph.x codes because I later<br></blockquote><blockquote type="cite">need to calculate third derivatives)<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">As regards the 72 atom supercell scf calculations not converging, the<br></blockquote><blockquote type="cite">corresponding input file is attached here. I increased the scf<br></blockquote><blockquote type="cite">convergence threshold to 1.0e-3 in order to terminate the computation<br></blockquote><blockquote type="cite">earlier (to check the results). I am attaching two output files-one<br></blockquote><blockquote type="cite">with threshold 1.0e-8 and the other with 1.0e-3. Both of them use a 72<br></blockquote><blockquote type="cite">atom supercell and a 2x2x1 kpoint grid. Please let me know if you<br></blockquote><blockquote type="cite">think tweaking a few parameters could help convergence.<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Thanks very much<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Dhruv<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">On Mon, Nov 22, 2010 at 8:26 AM, Paolo Giannozzi <<a href="mailto:giannozz@democritos.it">giannozz@democritos.it</a>> wrote:<br></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">On Nov 22, 2010, at 14:23 , Gabriele Sclauzero wrote:<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><blockquote type="cite">Hence, if you want more precision you probably need to use<br></blockquote></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><blockquote type="cite">better k-point sampling and C-C distances.<br></blockquote></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">and better (i.e. smaller) self-consistenty convergence thresholds.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">The error on the energy is a quadratic function of the error on the<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">charge density due to imperfect self-consistency; the error on forces<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">is instead linear.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">P.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">---<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">Paolo Giannozzi, Dept of Chemistry&Physics, Univ. Udine<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">via delle Scienze 208, 33100 Udine, Italy<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">Phone +39-0432-558216, fax +39-0432-558222<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">_______________________________________________<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">Pw_forum mailing list<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><a href="http://www.democritos.it/mailman/listinfo/pw_forum">http://www.democritos.it/mailman/listinfo/pw_forum</a><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">--<br></blockquote><blockquote type="cite">Dhruv<br></blockquote><blockquote type="cite"><br></blockquote><br><br><br>-- <br>Dhruv<br>_______________________________________________<br>Pw_forum mailing list<br><a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a><br>http://www.democritos.it/mailman/listinfo/pw_forum<br></div></blockquote></div><br><div>
<span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: medium; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div><span class="Apple-style-span" style="color: rgb(126, 126, 126); font-size: 16px; font-style: italic; "><br class="Apple-interchange-newline">§ Gabriele Sclauzero, EPFL SB ITP CSEA</span></div><div><font class="Apple-style-span" color="#7E7E7E"><i> PH H2 462, Station 3, CH-1015 Lausanne</i></font></div></span>
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