[Pw_forum] The choice of Acoustic Sum Rule for crystal
Nicola Marzari
marzari at MIT.EDU
Wed Jan 17 12:32:47 CET 2007
The difference for the high frequency modes between the two sum rules
is somewhat suspicious. I would try to increase the k-point sampling,
and see if this improves agreement.
Also, make sure that your sfc, nscf and phonon calculations are all
well converged. Tighten the tolerances for all, and see if anything changes.
nicola
naivebamboo Huang wrote:
> 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
>>>
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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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Prof Nicola Marzari Department of Materials Science and Engineering
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