[Pw_forum] The choice of Acoustic Sum Rule for crystal

[Matteo Cococcioni]

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
>
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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
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