[Pw_forum] The choice of Acoustic Sum Rule for crystal
naivebamboo Huang
naivebamboo at hotmail.com
Tue Jan 16 06:16:49 CET 2007
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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