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 ----------------------------------------------------------------------------------------------------------------- -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20140919/84e3c850/attachment.html -------------- next part -------------- A non-text attachment was scrubbed... Name: image.png Type: image/png Size: 15989 bytes Desc: not available Url : http://pwscf.org/pipermail/pw_forum/attachments/20140919/84e3c850/attachment.png
