[Pw_forum] Phonon calculation, but inexact atomic coordinates

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Dear Stefano,

Thank you very much for your kind reply. But, as you
said,

> 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. 

Did you mean, even if the input atomic coordinates are
not exactly the "real" equilibrium values, errors in
atomic positions will not cause any extra frequencies
error, besides the "standard" DFT error(e.g., k
sampling, finite cutoff energy, single particle
scheme...)? 

Or inexact atomic positions in UNPERTURBed geometry DO
cause errors in calculated frequencies?

Let me raise up the question in another way. As I
know, errors in lattice constants will affect the
accuracy of freqencies, because modification on
lattice constants means applying external pressure. I
think errors caused by inexact lattice constants are
easy to understand.

But, if the lattice constants are correct, but atomic
positions in the unit cell has some errors(just like
the case in carbon nanotubes), do this kind of errors
cause inaccuracy in frequencies, and, if yes(which I
believe), how serious will the error be?

Best wishes,

Linhui

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