[Pw_forum] Optical phonon convergence at Gamma-point

[Sophia Nishad]

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
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