[Pw_forum] phonon dispersion with experimental structure

[Stefano de Gironcoli]

the phonon code calculate the second order derivative of the energy at 
the given configuration.
this can be done even the first order derivatives are non vanishing.
provided the structure is not too far from the minimum the computed 
curvature can still quite close to the
one one could have computed at equilibrium but it is an approximation to it.

stefano

On 05/01/2013 05:18 PM, ding wrote:
> Dear everyone,
>    Does one have to relax the structure before doing phonon calculations?  If I calculate the phonon dispersion of a material with the experimental structure at different pressures, Can I get get the phonon dispersions without negtive frequencies at different pressures ? I know that one has to optimize the structure before doing phonon calculation when using the small displacement method, but I am not sure if one has to optimize the structure before doing phonon calculations when using linear response  theory. I want to calculate  the electron-phonon interaction properties with the experimental structures at different pressures. If I don't optimize the structures, Can I get the phonon dispersions without negtive frequencies ?
> Thanks in advance!
>
>
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