[Pw_forum] Phonon calculation, but inexact atomic coordinates

[Stefano de Gironcoli]

fdsa fda wrote:

> > DFPT compute the phonon frequency from the curfature
> > of the total energy with respect to atomic
> > positions. The fact that the atomic configuration is
> > not exactly the equilibrium one will modify the
> > curvature but should not induce additional errors.
> >
>
> I'm sorry to find the above words hard to understand.

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