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 -------------- next part -------------- A non-text attachment was scrubbed... Name: fit.eps Type: application/postscript Size: 14786 bytes Desc: Url : http://www.democritos.it/pipermail/pw_forum/attachments/20071212/c72f182f/attachment-0001.eps
