[Wien] NaCl Phonon calculations

[Peter Blaha]

This is NaCl, thus no need (no possibility) to optimize positions.

Of course the volume affects the phonon frequencies, but unless you
have a very unreasonable volume (mixed Ang and bohr ????) the NaCl
structure should be stable and thus imaginary frequencies should not appear.

I'd expect you made an error during init_phonon. Remember, you must not accept
changes by sgroup (when it wants to change the lattice), but have to find the
proper symmetry by nn.

Maybe try first a 1x1x1 supercell. Of course in this case the supercell is
too small, but still at Gamma and X (?) the frequencies should be exact.

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

                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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