[Pw_forum] Zn NC_PP from qe-forge

[William Parker]

Thanks, Eric, for the insight.  That core correction can be vicious.  Turning up the density cutoff does help.  There is still oscillation in the total energy at the 100 micro-Ry (meV) level for density cutoff 16 times the wave function cutoff, but I guess one shouldn't expect too much stability at that level.

https://wiki.alcf.anl.gov/wparker/images/6/68/ZnO_ecut_convergence_rho4_10_16_lda_ncpp.png

--William
		
On Oct 19, 2011, at 9:13 PM, Eric J. Walter wrote:

> 
> Hi,
> 
>> Just to double-check. Is charge density cutoff 4 times that planewave cutoff?
> 
> I think that this hits on the issue...  The Zn.pz-nc.UPF has a sharp 
> core correction.  The FFT/Density grid
> needs to be increased significantly, here is a plot of from 4x to 20x 
> ecutwfc for the case with and without
> a core correction in Zn. I think this is the cause of the odd *ecutwfc* 
> shown by William Parker.
> 
> http://dl.dropbox.com/u/8094696/Znplot.pdf
> 
> So, in this case, no, the density cutoff needs to be higher:
> 
> 
> Eric
> 
> 
> 
> 
> On 10/19/2011 08:47 PM, Nichols A. Romero wrote:
>> The behavior here is what I might expect for a real-space DFT code,
>> but V_NL is evaluated in reciprocal space right (we don't have
>> real-space projectors)?
>> 
>> Just to double-check. Is charge density cutoff 4 times that planewave cutoff?
>> 
>> 
>> On Wed, Oct 19, 2011 at 2:51 PM, William Parker<wparker at anl.gov>  wrote:
>>> 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
>>> 
>>> *********************************************************
>>>  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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>> 
>> 
> 
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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
*********************************************************