[Pw_forum] missing imaginary frequencies

[Andrea Salguero]

Dear Paolo and Serge,
thank you very much for your reply, that's it, in order to make the 
dispersion calculation I used Z=0,0,1/2 and used the same for the 
individual k-point calculation of my tetragonal system. After trying 
with z=0,0,c/a I got the correct results.
Best Regards,
andrea


Serge Nakhmanson wrote:
> My advice would be to recheck, for an individual k-point calculation,
> that the point you think is Z is actually Z in QE's units, which are
> *not* crystal units. E.g., for a tetragonal P4mm system with c/a != 1,
> Z will not be [0,0,1/2], but something else, depending on the c/a ratio.
>
> This problem does not come up in phonon-dispersion calculations, since
> there the k-point grid is generated automatically (and correctly, as
> one would hope), so that Z is always what it should be.
>
> S.
>
> Paolo Giannozzi wrote:
>   
>> On Jan 17, 2011, at 20:02 , Andrea Salguero wrote:
>>
>>     
>>> the problem is not in the phonon dispersion but in the calculation  
>>> of the
>>> frequency at Z point, other calculations show there is an  
>>> instability at that
>>> point, which is observed in the phonon dispersion I have obtained  
>>> with PWscf
>>> code as well. However, I don't get imaginary frequencies when  
>>> calculating the
>>> frequencies only at that point.
>>>       
>> very strange. If the Z point is in the wavevector grid used to calculate
>> interatomic force constants in real space, you should get the same
>> results from the single and the complete calculation. If not, there can
>> be a small difference, but nothing more than a small difference, if
>> everything is properly done
>>
>> Paolo
>>     
>
>
>