[Pw_forum] Phonon calculation, but inexact atomic coordinates

[fdsa fda]

Dear Stefano,

This time I've got it. Thank you very much!

Best wishes,


Linhui


> I wanted to say that the presence of non zero forces
> on the atoms
> in a non equilibrium structure do not create a
> problem in the
> calculation of the frequencies.
> 
> Obviously if the the position are slightly different
> from
> the equilibrium ones there will be an effect on the
> frequencies.
> 
> I would expect that the change in the frequencies
> due to "random"
> 1 % stretching (or compression)  of the bonds will
> be of the order of
> what one obtains by stretching coherently all the
> bonds when
> expanding the cell. Therefore of the order of a few
> %.
> There might be some frequencies that are determined
> by some local
> environment which will be more sensittive to
> somespecific atomic
> positions but as a rule of tumb I would expect that
> an estimate based
> on the  gruneisen parameters should be qualitatively
> correct.
> 
> regards
> 
> stefano
> 
> 
> 
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