<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Dear all: two further comments.<div><br class="webkit-block-placeholder"></div><div>1) I have not seen in detail the output of Xu Yuehua's calculations, but I would like to point out that one may have negative frequencies even if the atomic forces and crystal stress vanish. Negative frequencies are a manifestation of a mechanical instability that may occur even when the first derivatives of the energy with respect to any possible mechanical distortion of the lattice vanish.</div><div><br class="webkit-block-placeholder"></div><div>2) I am not myself a native English speaker, and I know by experience how much it takes to write in a foreign language. I am also aware that the difficulty of writing decent English depends a lot on one's own mother tongue. However, I would like to kindly urge contributors to this mailing list to read twice their posts and always ask themselves very carefully if what they have written can be understood with a fair investment of time and attention. Working on one's own way of expressing oneself is a very important (and rewarding, rest assured!) exercise that non-native English speakers should do.</div><div><br class="webkit-block-placeholder"></div><div>Best wishes,</div><div>SB</div><div><br><div><div>On Dec 12, 2007, at 12:11 PM, Hande Ustunel wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">Dear Xu Yuehua,<br><br>You should fit your energies to a parabola and find the minimum of the<br>fit. For a second order fit, your minimum is at about 2.83. 2.77 is just a<br>local blip in your rather jumpy data. You cannot decide by looking at the<br>individual energies because they do not follow a smooth function. But even<br>for the "wrong" lattice constant, there's no physical reason why you should<br>get negative frequencies if you have converged your electronic and atomic<br>coordinates to high accuracy. I'm attaching the fit as an example.<br><br>I would also suggest increasing your cutoff a little because your data, as<br>I said, is a bit noisy near the minimum.<br><br>Best,<br>Hande<br><br>On Wed, 12 Dec 2007, xu yuehua wrote:<br><br><blockquote type="cite">hi ,uesers here :<br></blockquote><blockquote type="cite">i relax one crystal lattice vs energy:<br></blockquote><blockquote type="cite">i have compute several energy list :<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">c E(RY)<br></blockquote><blockquote type="cite">2.795 -137.79266<br></blockquote><blockquote type="cite">2.78 -137.79248<br></blockquote><blockquote type="cite">2.77 -137.79233<br></blockquote><blockquote type="cite">2.8 -137.7927<br></blockquote><blockquote type="cite">2.81 -137.79275<br></blockquote><blockquote type="cite">2.82 -137.79275<br></blockquote><blockquote type="cite">2.83 -137.79278<br></blockquote><blockquote type="cite">2.84 -137.79276<br></blockquote><blockquote type="cite">2.85 -137.79279<br></blockquote><blockquote type="cite">2.86 -137.79272<br></blockquote><blockquote type="cite">2.87 -137.79265<br></blockquote><blockquote type="cite">2.88 -137.79258<br></blockquote><blockquote type="cite">2.9 -137.79229<br></blockquote><blockquote type="cite">2.95 -137.79122<br></blockquote><blockquote type="cite">these energy points are smooth and can be plotted parabola.<br></blockquote><blockquote type="cite">the system has 12 atoms.the energy is different most at the third digit<br></blockquote><blockquote type="cite">after radix point. so i think the average energy/atom is less 0.0001.<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">i want to ask the within the precision of energy(my thinking is based on<br></blockquote><blockquote type="cite">that :my above calculation is set ecutwfc:29, ecutrho:200,and the criterion<br></blockquote><blockquote type="cite">is 35,350, the convergence is 0.0049345 ry.)<br></blockquote><blockquote type="cite">so this small difference of energy can skip,i.e we can choose 2.77 which is<br></blockquote><blockquote type="cite">regarded as the best structure?<br></blockquote><blockquote type="cite"> as why i ask this question,for the the structure of 2.85 has two big<br></blockquote><blockquote type="cite">negative frequencies near -100cm-1, even if i add asr .<br></blockquote><blockquote type="cite">but 2.77 has no negative frequency after i add asr.<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"><br></blockquote><br>-- <br>Hande Ustunel<br>Department of Physics<br>Office 439<br>Middle East Technical University<br>Ankara 06531, Turkey<br>Tel : +90 312 210 3264<br><a href="http://www.physics.metu.edu.tr/~hande">http://www.physics.metu.edu.tr/~hande</a><br><span><fit.eps></span>_______________________________________________<br>Pw_forum mailing list<br>Pw_forum@pwscf.org<br>http://www.democritos.it/mailman/listinfo/pw_forum<br></blockquote></div><br><div> <span class="Apple-style-span" style="border-collapse: separate; 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