<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 19.18, Dhruv Singh ha scritto:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div>Thanks very much for the help regarding this topic.... I guess the<br>hexagonal supercell is a better choice then and I will need to use a<br></div></blockquote><div><br></div><div>If there's no specific reason for using a lower symmetry cell (as the tetragonal one you've been using previously), I think so.</div><br><blockquote type="cite"><div>finer k point grid with tighter scf convergence criterion for better<br>accuracy in forces. As an advice though, would you suggest that for<br>subsequent force constant calculations - forces of the order of 1.0e-5<br></div></blockquote><div><br></div><div>That's a very tight threshold! i.e. 2 orders of magnitudes smaller than the default one...</div><br><blockquote type="cite"><div>would not lead to unstable modes ? (I am using a supercell to<br>calculate force constants rather than the ph.x codes because I later<br>need to calculate third derivatives)<br><br>As regards the 72 atom supercell scf calculations not converging, the<br>corresponding input file is attached here. I increased the scf<br>convergence threshold to 1.0e-3 in order to terminate the computation<br>earlier (to check the results). I am attaching two output files-one<br>with threshold 1.0e-8 and the other with 1.0e-3. Both of them use a 72<br>atom supercell and a 2x2x1 kpoint grid. Please let me know if you<br>think tweaking a few parameters could help convergence.<br></div></blockquote><div><br></div><div>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).</div><div><br></div><div>HTH</div><div><br></div><div>GS</div><br><blockquote type="cite"><div><br>Thanks very much<br><br>Dhruv<br><br>On Mon, Nov 22, 2010 at 8:26 AM, Paolo Giannozzi <<a href="mailto:giannozz@democritos.it">giannozz@democritos.it</a>> wrote:<br><blockquote type="cite"><br></blockquote><blockquote type="cite">On Nov 22, 2010, at 14:23 , Gabriele Sclauzero wrote:<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"><blockquote type="cite">Hence, if you want more precision you probably need to use<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">better k-point sampling and C-C distances.<br></blockquote></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">and better (i.e. smaller) self-consistenty convergence thresholds.<br></blockquote><blockquote type="cite">The error on the energy is a quadratic function of the error on the<br></blockquote><blockquote type="cite">charge density due to imperfect self-consistency; the error on forces<br></blockquote><blockquote type="cite">is instead linear.<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">P.<br></blockquote><blockquote type="cite">---<br></blockquote><blockquote type="cite">Paolo Giannozzi, Dept of Chemistry&Physics, Univ. Udine<br></blockquote><blockquote type="cite">via delle Scienze 208, 33100 Udine, Italy<br></blockquote><blockquote type="cite">Phone +39-0432-558216, fax +39-0432-558222<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">_______________________________________________<br></blockquote><blockquote type="cite">Pw_forum mailing list<br></blockquote><blockquote type="cite"><a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a><br></blockquote><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 type="cite"><br></blockquote><br><br><br>-- <br>Dhruv<br><span><gra.scf.in></span><span><gra2by2_thr1.0e-3.scf.out></span><span><gra2by2_thr1e-8.scf.out></span>_______________________________________________<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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