[Pw_forum] PAW potentials and ecutrho convergence

[Joshua Davis]

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:
[image: 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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