Dear QE users,
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).
The proposed procedure is as follows:
1. Use the cif2cell software package to create a template QE input of the
anatase bulk from the anatase CIF
2. Run convergence tests on this bulk structure (i.e. perform several SCF
calculations on the structure while varying the given parameter):
* Ecutwfc
* Ecutrho (I'm using PAW pseudopotentials, so the standard ecutwfc x 4
is not sufficient)
* K-points
* Cell parameters
3. Optimize structure with optimized parameters to get final bulk structure
4. Convert QE output to CIF
5. Use the cif2cell software package to create QE input for a (101) slab
created from the optimized bulk structure CIF
6. Freeze bottom layer in QE input
7. Run convergence tests on slab structure:
* Vacuum width
* Layer thickness
My specific questions for this procedure are the following:
1. 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?
2. 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?
3. Is the order of the convergence tests on the bulk structure right, or
should I perform them in another order?
4. 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?
5. 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.
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.
Thanks in advance and kind regards,
Leon Luntadila Lufungula
PhD student at Structural Chemistry Group
University of Antwerp
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