[Pw_forum] PAW potentials and ecutrho convergence

[Paolo Giannozzi]

Maybe the nonlinear core correction is slowly converging,
especially in Sodium which has s and p semicore electrons 
and a very localized (pseudo-)core charge. Just guessing.

P.

On Fri, 2014-09-19 at 14:20 -0400, Joshua Davis wrote:
> I was performing a convergence test for the energy cut-off for the
> density for a set of PAW potentials just to make sure, but I have some
> results that confuse me a bit.
> 
> 
> The compound is a NaB5C (space group: Pm-3m) so the potentials I used
> were:
>  
> B.pbe-n-kjpaw_psl.0.1.UPF  
> C.pbe-n-kjpaw_psl.0.1.UPF  
> Na.pbe-spn-kjpaw_psl.0.2.UPF
> 
> 
> The cell was also completely relaxed each time with cell and ion
> dynamic being "bfgs" algorithm 
> 
> 
> 
> Already did a ecutwfc and kpoint convergence test for each and I found
> and ecutwfc of 35 Ry and a kpoint mesh of 4x8x8 to be sufficient (I am
> calculating a cell doubled in one direction).  
> 
> 
> When I did the ecutwfc convergence test I went from 10 to 50 Ry in
> increments of 5 Ry, and I kept the default ecutrho=4*ecutwfc for each
> run.
> 
> 
> so when I did the ecutrho convergence kept ecutwfc at 35 Ry and the
> kpoints at 4x8x8 and started at 140 and went to 400 Ry.
> 
> 
> The graph of my convergence is shown below:
> Inline image 1
> 
> 
> and if the fourm scrubs the image here is the raw data:
> ecutrho(Ry),TotalEnergy(ry)
> 140,-370.0979858
> 150,-370.0980023
> 160,-370.0995505
> 170,-370.1019804
> 180,-370.1047998
> 190,-370.1072663
> 200,-370.1086284
> 250,-370.1048218
> 300,-370.1049835
> 400,-370.1060986
> 
> 
> 
> 
> The convergence shows a density cut-off of 140 Ry and 150 Ry to be
> pretty similar in calculated energy, but after that the calculated
> energy quickly drops until a cut-off of 200 Ry.  After that the
> calculated energy shoots up at a cut-off of 250 Ry by 0.004 then
> tappers down  by 0.002 Ry as it gets to a cut-off of 400 Ry.
> 
> 
> I thought the energy cut-off for density would be the easiest after
> ecutwfc and kpoint, but I want to make sure my results would be fine
> based on what I read for he input documentation for ecutrho.  It says
> ecutrho=4*ecutwfc is usually good (which is pretty standard for the
> PAW method), but I wanted to make sure.  I thought the calculated
> energy would not have changed or the calculated energy would have
> converged at a reasonable value.
> 
> 
> Am I doing something wrong? Is the ratio of ecutwfc and ecutrho too
> off at the higher energy ecutrho cut-off?  Other posts suggest
> lowering ecutwfc and keeping ecutrho cut-off constant for proper
> convergence, should I have done this?  I figured sneaking up ecutrho
> would be okay.  Am I dumb and this is just an artifact of the PAW
> method at a large ratio of ecutwfc to ecutrho?  Is this an artifact of
> how the FFT mesh is implemented in QE 5.1?  Should I have not gone
> that high in my density cut-off?  
> 
> 
> Any help would be much appreciated. 
> 
> 
> Other considerations:
> I used QE 5.1 compiled with ifort 13 and mkl 11.  
> 
> 
>  &CONTROL
>    calculation = 'vc-relax',
>    etot_conv_thr = 1.0D-5,
>    forc_conv_thr = 1.0D-4
>  /
> 
> 
>  &SYSTEM
>    ibrav = 0,
>    nat = 14,
>    ntyp = 3,
>    nosym = .TRUE.,
>    nspin = 1,
>    occupations = 'fixed',
>  
> ----------------------------------------------------------------------------------------------------------------
> Joshua D. Davis
> 
> Graduate Assistant
> Department of Chemistry
> Michigan State University
> 578 S. Shaw Lane, room 432
> 
> 
> East Lansing, MI 48824
> 
> -----------------------------------------------------------------------------------------------------------------
> 
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