[Pw_forum] Convergence of Phonon with K points

[Prasenjit Ghosh]

Ciao Stefano,

Thanks for a clear explanation.

Prasenjit

On 10 August 2011 18:00, Stefano Baroni <baroni at sissa.it> wrote:
> 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,
> SB
>
> 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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-- 
PRASENJIT GHOSH,
Assistant Professor,
IISER Pune,
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