[Wien] NaCl Phonon calculations

[Marco Schowalter]

Sergio Yanuen Rodriguez wrote:

>Dear Wien2k users
>
>I am trying to do phonon calculations for NaCl because this system is
>already studied and after this apply this method to something more
>complicated, I generate the case.d45 with PHONON for a 2X2X2 supercell,
>then I transfer this file to Wien 2k and calculate the forces, but when I
>run analyse_phonon the sum of this forces are not small, and the results I
>get do not seem alright. I compared this results with another ones and
>they do not agree specially for the acoustic modes. Since I am getting
>negative frequencies at Gamma where they must have 3 (nearly) zero
>frequencies.
>
>Does any one know which parameter is important to adjust to make the sum
>of the forces small? or do I have to do a volume optimization before
>calculating the forces? Or any suggestions
>
>  
>
Dear Sergio,

one should do a volume optimization before computing the 
Hellmann-Feynman forces. If the negative frequencies phonon frequencies 
at gamma are not too small (nearly zero) they may be due to some 
numerical errors. You can then impose the translation-rotational 
invariances (in create/hellmann-feynman f./trans.rotInv.). From my 
experience a 2x2x2 supercell should be ok. Do  you use LDA or GGA? Did 
you displace the atoms in positive and negative direction ? If you did 
not use optimized lattice parameter and did not displace in positive and 
negative direction, then I would try first to optimize and then displace 
in positive and negative direction.

I hope this helps

Marco





>I really appreciate any help.
>
>Sergio Y. Rodriguez
>Physics Department
>Texas A&M University
>
>
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>  
>


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Dr. Marco Schowalter
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Email: schowalter at ifp.uni-bremen.de

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