[Pw_forum] about phonon calculation

[Stefano de Gironcoli]

sun jason wrot

>
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>
> and the frequency at gamma point is like this:
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>
> 0.0000 0.0001 0.0001 1055.5528 1055.5528 1274.3693
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>
>
> so my question is:
>
> 1)why the LO mode is so small(the exact value is about 1305 cm-1) when
> the TO mode is correct? and how to improve it? 

Has the lattice parameter been optimized ? is 50 ryd cutoff sufficient ?
is LDA adequate in this case ?
is 30 cm-1 out of 1300 really bad compared with present
experimental/theoretical accuracy ?

>
> 2) why the absolute value of effective charge for B atom and N atom is
> not equal?
>
because the ASR is only exactly satisfied in calculations with very
large k-point samplig.
The violation you report is not large and in any case you can impose ASR
in the input of dynmat.x obtaining results equivalent to calculations
with better sampling.

stefano de Gironcoli