Dear all: two further comments. 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.
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. Best wishes, SB On Dec 12, 2007, at 12:11 PM, Hande Ustunel wrote: > Dear Xu Yuehua, > > You should fit your energies to a parabola and find the minimum of the > fit. For a second order fit, your minimum is at about 2.83. 2.77 is > just a > local blip in your rather jumpy data. You cannot decide by looking > at the > individual energies because they do not follow a smooth function. > But even > for the "wrong" lattice constant, there's no physical reason why you > should > get negative frequencies if you have converged your electronic and > atomic > coordinates to high accuracy. I'm attaching the fit as an example. > > I would also suggest increasing your cutoff a little because your > data, as > I said, is a bit noisy near the minimum. > > Best, > Hande > > On Wed, 12 Dec 2007, xu yuehua wrote: > >> hi ,uesers here : >> i relax one crystal lattice vs energy: >> i have compute several energy list : >> >> c E(RY) >> 2.795 -137.79266 >> 2.78 -137.79248 >> 2.77 -137.79233 >> 2.8 -137.7927 >> 2.81 -137.79275 >> 2.82 -137.79275 >> 2.83 -137.79278 >> 2.84 -137.79276 >> 2.85 -137.79279 >> 2.86 -137.79272 >> 2.87 -137.79265 >> 2.88 -137.79258 >> 2.9 -137.79229 >> 2.95 -137.79122 >> these energy points are smooth and can be plotted parabola. >> the system has 12 atoms.the energy is different most at the third >> digit >> after radix point. so i think the average energy/atom is less 0.0001. >> >> i want to ask the within the precision of energy(my thinking is >> based on >> that :my above calculation is set ecutwfc:29, ecutrho:200,and the >> criterion >> is 35,350, the convergence is 0.0049345 ry.) >> so this small difference of energy can skip,i.e we can choose 2.77 >> which is >> regarded as the best structure? >> as why i ask this question,for the the structure of 2.85 has two big >> negative frequencies near -100cm-1, even if i add asr . >> but 2.77 has no negative frequency after i add asr. >> >> >> > > -- > Hande Ustunel > Department of Physics > Office 439 > Middle East Technical University > Ankara 06531, Turkey > Tel : +90 312 210 3264 > http://www.physics.metu.edu.tr/~hande > <fit.eps>_______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum --- Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype) Please, if possible, don't send me MS Word or PowerPoint attachments Why? See: http://www.gnu.org/philosophy/no-word-attachments.html -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20071213/265bb957/attachment.htm
