[QE-users] How to handle slab calculations

[Léon Luntadila Lufungula]

Dear QE users,

I have been performing calculations on surface-modified TiO2 for some time now, as I took over a research project from a previous PhD student who quit his PhD. The project was already running for some time so I based my calculations on his input files as I started to learn how to use QE. Now I’m reading a lot of stuff online about performing QE calculations and I start to notice that my input files might be wrong...

I am performing calculations on a 3-layer anatase (101) slab with a 20 Å vacuum width above it and adsorbing different adsorbates onto the surface. I just found out that apparently when you are adsorbing molecules onto a slab you should use a dipole correction (dipfield = .TRUE.), which I did not do... I also read that for slab calculations it is recommended to use the assume_isolated keyword to cut off the Coulomb interaction in the z-direction and that the dipfield and assume_isolated keyword should not be used simultaneously.

I tried reading the papers on dipfield and the different assume_isolated options but I’m still pretty new to QE and periodic calculations so I found them hard to understand completely... Therefore, I would be most grateful if somebody could elaborate on the following questions:


  1.  Is it correct that you should use either dipfield or assume_isolated and not both at the same time? If so, which one is the best one to use?
  2.  I don’t quite understand why you have to do a dipole correction when adsorbing a molecule on the surface, is this just due to the fact that you induce a dipole upon binding and the PBC enlarge this effect?
  3.  If assume_isolated is the way to go, which option should I use? From what I understand, I cannot use Makov-payne as it is “ONLY FOR CUBIC SYSTEMS” and my cell is monoclinic (a=b≠c, α=ß=90°≠γ), esm is also not an option as it “Requires cell with a_3 lattice vector along z, normal to the xy plane, with the slab centered around z=0.” which is not the case for my system. So this leaves 2D and martyna-tuckerman as possible options, but I can’t seem to find a straight answer which is best or what the differences are.
  4.  What about the calculations I have already performed? Are they useless without applying a dipole and/or coulomb correction?

Thanks in advance!

Léon Luntadila Lufungula
Master student
University of Antwerp


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