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<p class="MsoNormal"><span lang="EN-US">Dear QE users,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I want to construct an anatase (101) slab starting from the experimental CIF file of anatase (obtained from the American Mineralogist Crystal Structure Database) but I am unsure of the correct procedure with the associated
convergence tests of the parameters of the QE calculation. I am basing my procedure on (sparsely documented) work performed by my predecessor with some adjustments based on stuff I read and learned online (including from the MaX School on Advanced Materials
and Molecular Modelling with QE organized by the QE developers). <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">The proposed procedure is as follows:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<ol style="margin-top:0cm" start="1" type="1">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span lang="EN-US">Use the cif2cell software package to create a template QE input of the anatase bulk from the anatase CIF<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span lang="EN-US">Run convergence tests on this bulk structure (i.e. perform several SCF calculations on the structure while varying the given parameter):<o:p></o:p></span></li><ol style="margin-top:0cm" start="1" type="a">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level2 lfo1"><span lang="EN-US">Ecutwfc<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level2 lfo1"><span lang="EN-US">Ecutrho (I’m using PAW pseudopotentials, so the standard ecutwfc x 4 is not sufficient)<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level2 lfo1"><span lang="EN-US">K-points<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level2 lfo1"><span lang="EN-US">Cell parameters<o:p></o:p></span></li></ol>
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span lang="EN-US">Optimize structure with optimized parameters to get final bulk structure
<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span lang="EN-US">Convert QE output to CIF<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span lang="EN-US">Use the cif2cell software package to create QE input for a (101) slab created from the optimized bulk structure CIF<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span lang="EN-US">Freeze bottom layer in QE input<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span lang="EN-US">Run convergence tests on slab structure:<o:p></o:p></span></li><ol style="margin-top:0cm" start="1" type="a">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level2 lfo1"><span lang="EN-US">Vacuum width<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level2 lfo1"><span lang="EN-US">Layer thickness<o:p></o:p></span></li></ol>
</ol>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">My specific questions for this procedure are the following:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<ol style="margin-top:0cm" start="1" type="1">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo3"><span lang="EN-US">Steps 2a-c: My predecessor stated that convergence is achieved once the total energy difference between two consecutive calculations is smaller than 1 kcal/mol,
but during the MaX School they did convergence tests and chose converged parameters when the total energy differed by much less than 1 kcal/mol (they said 1 meV, so around 0.023 kcal/mol). Is 1 kcal/mol enough, or should I go lower?
<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo3"><span lang="EN-US">Step 2d: My predecessor did a vc-relax run to optimize the cell parameters, but in the MaX School I saw they optimized alat instead of performing a vc-relax run.
Which of the two is correct?<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo3"><span lang="EN-US">Is the order of the convergence tests on the bulk structure right, or should I perform them in another order?<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo3"><span lang="EN-US">Step 7a: I’m intending to investigate adsorbates (not adatoms, but small molecules) on the surface, so should I do the convergence tests of the vacuum width on a
clean surface or on a surface with a model adsorbate to be certain the actual vacuum is large enough to avoid interactions between the adsorbate and the adjacent slab?<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo3"><span lang="EN-US">Step 7b: I’m uncertain how to perform the convergence test for this parameter. I have read one option where you look at the difference in total energy between two
consecutive structures and state convergence once this difference is constant within chemical accuracy (1 kcal/mol), but I’m not certain if this is the correct way.<o:p></o:p></span></li></ol>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I just want to make sure I do this correctly, so I can start performing calculations without having to worry that my data is inaccurate. Hopefully someone can help me find the correct procedure.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Thanks in advance and kind regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Leon Luntadila Lufungula<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">PhD student at Structural Chemistry Group<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">University of Antwerp<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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