<div dir="ltr">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.<div><br></div><div>The compound is a NaB5C (space group: Pm-3m) so the potentials I used were:</div><div> </div><div>B.pbe-n-kjpaw_psl.0.1.UPF </div><div>C.pbe-n-kjpaw_psl.0.1.UPF </div><div>Na.pbe-spn-kjpaw_psl.0.2.UPF</div><div><br></div><div>The cell was also completely relaxed each time with cell and ion dynamic being "bfgs" algorithm <br></div><div><br></div><div>Already did a ecutwfc and kpoint convergence test for each and I found</div><div>and ecutwfc of 35 Ry and a kpoint mesh of 4x8x8 to be sufficient (I am calculating a cell doubled in one direction). </div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>The graph of my convergence is shown below:</div><div><img src="cid:ii_1488ee62c68ad307" alt="Inline image 1" width="483" height="291"></div><div><br></div><div>and if the fourm scrubs the image here is the raw data:</div><div>ecutrho(Ry),TotalEnergy(ry)</div><div><div>140,-370.0979858</div><div>150,-370.0980023</div><div>160,-370.0995505</div><div>170,-370.1019804</div><div>180,-370.1047998</div><div>190,-370.1072663</div><div>200,-370.1086284</div><div>250,-370.1048218</div><div>300,-370.1049835</div><div>400,-370.1060986</div></div><div><br></div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>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? </div><div><br></div><div>Any help would be much appreciated. </div><div><br></div><div>Other considerations:</div><div>I used QE 5.1 compiled with ifort 13 and mkl 11. </div><div><br></div><div><div> &CONTROL</div><div> calculation = 'vc-relax',</div><div> etot_conv_thr = 1.0D-5,</div><div> forc_conv_thr = 1.0D-4</div><div> /</div><div><br></div><div> &SYSTEM</div><div> ibrav = 0,</div><div> nat = 14,</div><div> ntyp = 3,</div><div> nosym = .TRUE.,</div><div> nspin = 1,</div><div> occupations = 'fixed',</div><div> </div></div><div><span style="font-family:arial,helvetica,sans-serif">----------------------------------------------------------------------------------------------------------------</span><br><div><div><div dir="ltr"><div style="margin-bottom:0px;margin-left:0px;padding-bottom:5px"><div style="overflow:hidden"><div dir="ltr"><font face="arial, helvetica, sans-serif">Joshua D. Davis</font></div><div dir="ltr"><font face="arial, helvetica, sans-serif"><br>Graduate Assistant</font></div><div dir="ltr"><div><span style="background-color:transparent"><font face="arial, helvetica, sans-serif">Department of Chemistry</font></span></div><div><span style="background-color:transparent"><font face="arial, helvetica, sans-serif">Michigan State University</font></span></div><div><span style="background-color:transparent"><p style="margin:0in 0in 0pt"><font face="arial, helvetica, sans-serif"><span lang="EN-US">578 S. Shaw Lane</span><u></u><u></u><span lang="EN-US">, room 432<u></u></span></font></p></span></div><div><font face="arial, helvetica, sans-serif"><span style="background-color:transparent">East Lansing, MI 48824</span><br></font></div><div><span style="background-color:transparent"><font face="arial, helvetica, sans-serif">-----------------------------------------------------------------------------------------------------------------</font></span></div></div></div></div><div style="font-size:13px"></div></div></div>
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