[Pw_forum] Zn NC_PP from qe-forge

[Eric J. Walter]

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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>
>