[Pw_forum] Zn NC_PP from qe-forge

[William Parker]

Dear Yun Song and interested parties,

The possibility of a phonon test case in ZnO piqued my curiosity so I decided to have a go at it.  I find a very odd convergence pattern with these two pseudopotentials in this system--the total energy has a damped sinusoidal decay with increasing cutoff:
(selecting the maxima and minima at 10 Ry sampling)

E_cutoff (Ry)	E_total (Ry)
100		-338.741
160		-338.660
210		-338.675
290		-338.665
370		-338.668
400		-338.667

Figure here: https://wiki.alcf.anl.gov/wparker/images/b/b5/ZnO_ecut_convergence_lda_ncpp.png
Details here: https://wiki.alcf.anl.gov/wparker/index.php/Zinc_Oxide_Parameter_Convergence

I have never encountered this kind of behavior with norm-conserving pseudopotentials before.  At worst, I've seen little steps in an otherwise smooth convergence to a value.  The eigenvalues at the various k-points don't appear to have any ghost states that come and go with cutoff.  Has anyone observed this kind of oscillation in the total energy with increasing plane-wave cutoff before?  What gives rise to it?

--William

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  William D. Parker                 
  Computational Postdoctoral Fellow   fax: (630) 252-4798
  MSD-212, Rm. C-215
  Argonne National Laboratory
  9700 S. Cass Ave.
  Argonne, IL 60439
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