[Pw_forum] Optical phonon convergence at Gamma-point

[Stefano Baroni]

On May 24, 2009, at 11:04 PM, Sophia Nishad wrote:

> Hi,

Sohia:

>  I have checked ecutrho as well (even though it is suggested in the  
> mailing list that the default relation for ecutrho should be ok with  
> norm-conserving pseudos).

in fact, with NC PPs there is no point in increasing the charge cutoff  
beyon 4 x wvfct cutoff

> With changes to the ecutrho (~8,10 etc.) I see small change to the  
> optical phonons' values (~1 cm-1), but it is still off from the  
> experiemntal value by ~8cm-1.

8/510 = 1.5% I highly value your expectations abouT DFT, linear  
response, QE, and all the rest, still 1.5% seems to me an excellent  
agreement with experiments


>  I was confused why no comment had been made in the link http://users.aims.ac.za/~paolo/tutorial_phon.htmat 
>  on the convergence of phonons with respect to ecutrho whereas the  
> importance of the effects of ecutwfc on PW calculations was  
> explained in so many places.
>
> Could I conclude that with the norm-conerving pseudo available on  
> the QE website it is not possible to get improved phonon result???
>
> It would be really helpful for me if someone could refer to results  
> for Si that had been done checking the convergence of phonons w.r.t  
> ecutwfc.

well done linear-response phonon calculations should just reproduce  
frozen-phonon calculations, to which all the explanations about total- 
energy calculations apply. there are residual subtelties regarding the  
fulfillment of the acoustic sum rule, but these mainly affect acoustic  
phonons. I will explain the little I have understood on this issue in  
another occasion.

SB

>


---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
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