Dear Baoling, I probably missed the point in the previous email. You probably meant that you are considering two different atomic positions for Te atoms along one direction at least. You are finding very close energies for the two positions, but they seem to be close too (depends on the cell parameter which you don't provide). Did you relax the structure? are they two local minima which you get with different initial structures (I don't think so) or one of them is slightly unstable towards the other? what are the atomic forces on each of them? What is the energy of a configuration with intermediate position of the Te atom?
hope this helps, Matteo On Jan 15 2007, naivebamboo Huang wrote: >Dear PWscf users, > I am trying to calculate the phonon eigen modes of Bi2Te3 crystal > at gamma point. The fraction of one Te atom has two different reported > values in the literature (x1=0.792, x2=0.7905). I want to know which > value is more appropriate. I calculated the energy of the cell and found > the configure with x=0.7905 has a very slightly lower energy(0.001% > lower, does this mean the well is flat?). I also calculated the > frequency. Both of the configurations give 14 positive frequencies and 1 > negative frequency (totally 15 modes). I just thought this may be due to > the acoustic sum rule. I impose asr='simple' and used dynmat.x to get the > frequency. Only x=0.792 gives 12 positive frequencies and 3 zero > frequency. But if I set asr='crystal', both configures can produce 12 > positive frequencies and 3 zero frequency. After checking with the gamma > point of experimental dispersion curve at 300 K, the configure x=0.792 > and asr='crystal' give the best results. My questions are > (1) which ASR should I use, since the different ASR give >different frequencies (10% difference for x=0.792 configure) > (2) how can I trust the results I calculated? Does the positive >eigen frequency indicate a stable structure? I guessed at 300 K, the >frequencies should be different from those of the relax structure. > >Thanks very much > >Baoling > >_________________________________________________________________ >The MSN Entertainment Guide to Golden Globes is here. Get all the scoop. >http://tv.msn.com/tv/globes2007/?icid=nctagline2 > >_______________________________________________ >Pw_forum mailing list >Pw_forum at pwscf.org >http://www.democritos.it/mailman/listinfo/pw_forum > -- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Matteo Cococcioni Department of Chemical Engineering and Materials Science University of Minnesota 151 Amundson Hall 421 Washington Av. SE Minneapolis, MN 55455 Tel. +1 612 624 9056 Fax +1 612 626 7246 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
