[Pw_forum] Convergence of Phonon with K points

Stefano Baroni baroni at sissa.it
Wed Aug 10 14:30:49 CEST 2011

Dear Mayank,

have you plotted the electronic energy bands? are you sure that the the system is not (semi-) metallic, possibly because of some shortcomings of the XC functional you are using?

more generally, imaginary frequencies signal a mechanical instability of your system. if the instability occurs at a finite wave-vector, that means that he system may undergo a transition to a structure whose (larger) unit cell is commensurate with the instability wave-vector. If the instability occurs at q=0, then some distortions of atomic positions may occur, which in turn may determine a distortion of the cell shape. If it is a sound velocity that vanishes, this finally indicates an elastic instability of the shape of the unit cell.

in any case, more insight is needed, which in turn would probably require more work from you. 
remember: no pain, no gain!

good luck,
On Aug 10, 2011, at 9:44 AM, mayank gupta wrote:

> Dear Sir
> It's magnetic oxide SrFeO2. Kohn anomaloy occurs in metallic system.
> Should I further increase k points or do something else.
> Thanks
>> it may depend on a Kohn anomaly (which are particularly sensitive to k-point sampling)
>> SB
>> On Aug 9, 2011, at 9:24 AM, mayank gupta wrote:
>>> Dear all
>>> I am doing phonon calculation of a magnetic system,  have not included
>>> any magnetic interaction on it. I am calculating phonon at  2x2x2 mesh
>>> (6 q points). I got the phonon frequency +ve at 5 q point  while at
>>> one q point it is negative. I have check this with 24 x24x24 k point
>>> and with very high ecut and observed  this particular phonon mode
>>> doesn't converge with k point while rest is converged. is it because
>>> of not taking in to account the magnetic part or something else.
> -- 
> Mayank
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Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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