[Pw_forum] Optical phonon convergence at Gamma-point

[Sophia Nishad]

Hi,

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). 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.

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.

Thanks,
Sophia.



On Sat, May 23, 2009 at 8:17 AM, Sophia Nishad <sophia.nishad at gmail.com>wrote:

> Hi QE users,
>
> I am a new user of QE. So, wanted to run some test cases for Si.  Energy
> calculations were ok, but phonons are not. I am having trouble to get good
> result of optical phonons for 2-atom Si. Convergence of energy is obtained
> with
>
> ecutwfc 25Ry
> k-mesh 6 6 6 0 0 0
> norm-conserving PP
> conv_thr =  1.0d-12
> tr2_ph=1.0d-20
>
> Even though the energy difference was reduced w.r.t. increasing ecutwfc and
> k-mesh, I could not find good values for optical phonons ((I did not worry
> about ASR for this Gamma calculation)) w.r.t these quantities. Here's what I
> get with 6x6x6 k-mesh without any offset of k-mesh:
>
> 15Ry    519.587327 [cm-1]
> 16Ry    516.231236 [cm-1]
> 20Ry    508.216250 [cm-1]
> 30Ry   510.229073 [cm-1]
> 35Ry   510.044651 [cm-1]
> 50Ry   509.893556 [cm-1]
> 100Ry  511.548608 [cm-1]
>
> So some oscillations are there. But I would say it is converged around 510.
> The experimental value is ~519. So the QE result is quite different.
>
> I read the tutorials and archieve, but I did not find the answer of my
> problem.
>
> http://users.aims.ac.za/~paolo/tutorial_phon.htmat
> obtained the optical phonons to be 516.197992 [cm-1] for a ecutwfc of 16Ry.
> But at this ecutwfc the energy is not converged for Si. If I find ecutwfc to
> get energy convergence it becomes above 20Ry. But at this ecutwfc optical
> phonons are not getting better. I would expect better results at higher mesh
> and cutoff. I checked my result adding epsil=.true. as well. But result
> didn't change much.
>
> Could you suggest what might improve the result.
>
> Thanks in advance,
> --
> Sophia
> Univ. of California - Berkeley, USA
>



-- 
Sophia
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