[Pw_forum] Phonon at q=0

[Stefano Baroni]

Try to increase the number of k points and let us know what happens. If, as I guess, the zone-center acoustic modes soften, try to understand why, starting from the early literature on DFPT. A good starting point could be e.g., http://link.aps.org/doi/10.1103/PhysRevB.33.7017 and http://link.aps.org/doi/10.1103/PhysRevLett.62.2853 .
Hope this helps
Stefano Baroni

On Jul 2, 2011, at 12:02 PM, mayank gupta wrote:

> Thanks Sir for your response.
> 
> I am doing all these calculations for oxides. for example I have done
> the phonon calculations for Cu2O and Ag2O system the details are as
> follow
> 
> Ag2O ecutwfc 70 Ry.  Mesh  (8x8x8)        zone ceneter acoustic
> mode=1.6 THz (with PBE)
> Cu2O ecutwfc  95 Ry. Mesh  (8x8x8)        zone centre acoustic mode =
> 1.33 THz (PW)
> 
> I have performed above calculation using tr2_ph=1.0e-14, but the
> values are almost same. the ecutrho=20times of ecutwfc.
> 
> 
> Mayank
> 
> 
>> Dear Mayanka,
>> 
>> is this a polar material? If so, the violation of the ASR may be justified
>> by the use of too coarse a k-point mesh. Please, provide more details on the
>> system and the calculations you are doing.
>> 
>> SB
>> 
>> On Jul 1, 2011, at 6:17 AM, mayank gupta wrote:
>> 
>>> Dear All
>>> I have a query ragrdin phonon calculations at q=0. When I performed
>>> the phonon caclualtion at q=0, we didn't get zero enrgy for acoustic
>>> branches, however they can be set to zero by using acoustic/ crystal
>>> sum rules. My question is if the zone center acoustic braches values
>>> are too large (4 THz) even we have implemented very high convergence,
>>> it is ok to apply ASR.
>>> 
>>> Thanks
>>> 
>>> --
>>> 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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