[Pw_forum] The choice of Acoustic Sum Rule for crystal

naivebamboo Huang naivebamboo at hotmail.com
Tue Jan 16 22:33:37 CET 2007


Dear Matteo,
    Thanks very much. Yes, they are fractional coordinates of one Te atom 
(Te1 atom), that is, atomic position in the unit cell. I am sorry for the 
confusion. What I mean is that the energies of the two positions seems to 
change little comparing to their position change. The lattice constant is 
10.45 angstrom, and so the difference of the position is not small. I think 
this is due to the weak Te1-Te1 interaction (van der Waals interaction). I 
tried to relax the structure use vc-relaxation, but meet an error during the 
optimization. I am trying to work it out now. The configure with x=0.7905 is 
the relaxed structure that other people get use the same pseudopotential. 
The frequencies I got from PWscf are attached.
     You can see mode 4 of the two configures are much different. Also, if 
ASR changes from 'simple' to 'crystal', 10% difference appears (for all the 
modes). Is this acceptable? Since the frequencies are all positive, I just 
guess they are at least local minimal configures. The configure with 
x=0.792, though energy is higher, reproduces the measured modes best. I 
don't know which result I should trust.

Thanks

Baoling

x=0.7905     total energy =-382.4400695 Ry
x=0.792       total energy =-382.43346308 Ry (however, x=0.792 reproduces 
the measured bond lengths of Te-Bi, Te-Te )

x=0.7905 ( ASR='crystal')
#  mode   [cm-1]     [THz]       IR
    1      0.00    0.0000    0.0000
    2      0.00    0.0000    0.0000
    3      0.00    0.0000    0.0000
    4     11.81    0.3541    0.0000
    5     79.89    2.3950    0.0000
    6     79.89    2.3950    0.0000
    7     84.52    2.5339    0.0000
    8     84.52    2.5339    0.0000
    9    101.25    3.0353    0.0000
   10    128.73    3.8593    0.0000
   11    142.63    4.2760    0.0000
   12    150.50    4.5118    0.0000
   13    150.50    4.5118    0.0000
   14    153.94    4.6148    0.0000
   15    153.94    4.6148    0.0000

x=0.792 (ASR='crystal')
#  mode   [cm-1]     [THz]       IR
    1      0.00    0.0000    0.0000
    2      0.00    0.0000    0.0000
    3      0.00    0.0000    0.0000
    4     36.49    1.0941    0.0000
    5     75.48    2.2627    0.0000
    6     75.48    2.2627    0.0000
    7     83.00    2.4884    0.0000
    8     83.00    2.4884    0.0000
    9    100.00    2.9978    0.0000
   10    111.66    3.3476    0.0000
   11    138.25    4.1446    0.0000
   12    141.25    4.2346    0.0000
   13    141.25    4.2346    0.0000
   14    167.48    5.0210    0.0000
   15    167.48    5.0210    0.0000

x=0.792 (ASR='simple')
#  mode   [cm-1]     [THz]       IR
    1      0.00    0.0000    0.0000
    2      0.00    0.0000    0.0000
    3      0.00    0.0000    0.0000
    4     53.43    1.6017    0.0000
    5     56.12    1.6825    0.0000
    6     56.12    1.6825    0.0000
    7     69.99    2.0982    0.0000
    8     69.99    2.0982    0.0000
    9     98.14    2.9421    0.0000
   10     98.14    2.9421    0.0000
   11    113.40    3.3997    0.0000
   12    113.40    3.3997    0.0000
   13    114.78    3.4411    0.0000
   14    131.36    3.9381    0.0000
   15    145.97    4.3760    0.0000

>From: Matteo Cococcioni <matteo at umn.edu>
>Reply-To: pw_forum at pwscf.org
>To: pw_forum at pwscf.org
>Subject: Re: [Pw_forum] The choice of Acoustic Sum Rule for crystal
>Date: 16 Jan 2007 12:14:01 -0600
>
>
>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. 
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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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>
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