[Pw_forum] Zn NC_PP from qe-forge

[Nichols A. Romero]

Sorry for replying to this old thread. I still don't quite understand
the behavior for the energy as a function E_{cut,psi} for the
case of E_{cut,rho}=4*E_{cut,psi}. You would expect for a
norm-conserving pseudopotential that the total energy be monotonically
decreasing (Rayleigh-Ritz principle) as it achieves convergence. But
adding the non-linear core correction (nlcc) seems to negate
this behavior. I am assuming that the nlcc can be evaluated exactly in
real-space. Perhaps the FFT is leading to some aliasing
effect?

On Thu, Oct 20, 2011 at 11:05 AM, William Parker <wparker at anl.gov> wrote:
> 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
> *********************************************************
>
>
>
>
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-- 
Nichols A. Romero, Ph.D.
Argonne Leadership Computing Facility
Argonne, IL 60490
(630) 447-9793