[QE-users] recommended methods for slab calculations

[Giuseppe Mattioli]

Dear Michal
I'm a fan of esm. If you want to study the adsorption of molecules  
inducing a strong vertical dipole you are forced to decouple such  
dipole along the z direction of the cell, otherwise you experiment an  
horrible "capacitor effect" on the potential curve. To use esm='bc1'  
(a vacuum-slab-vacuum geometry) you need to confine the electronic  
density of your system (the density, not the atoms...) between -l/4  
and l/4 (you can shift the 0 of the cell if you don't want to shift  
the atoms), where l is the z edge of your supercell. I don't know if  
you consider this as an "enormous vacuum padding"...

Moreover, esm produces "for free" a very useful file (in the TMP_DIR),  
namely prefix.esm1, where at the end of each scf iteration useful  
quantities are plotted, such as the electrostatic potential and the  
charge density integrated on x,y planes. You may want to have a look  
to this recent paper (J. Phys. Chem. C 2020, 124, 3601) to see what  
you can do with such data (e.g., calculate changes in the surface  
workfunction upon the adsorption of polar molecules).

Finally, during the first scf iteration you may experiment a bit of  
instability, but it is generally not a dramatic problem...

HTH
Giuseppe


Quoting Michal Krompiec <michal.krompiec at gmail.com>:

> Hello,
> I was wondering which assume_isolated method is the preferred choice
> for calculation of adsorption energies of small molecules on 2D slabs
> of metals and oxides.
> So far, I've been using assume_isolated='2d', but it requires an
> enormous vacuum padding. Is esm less resource-hungry? Does anyone use
> a less accurate scheme first (e.g. no "assume_isolated" and a small
> vacuum padding) and moves later to the target large cell with
> assume_isolated=2d?
> I understand that it all depends on the system, and that a convergence
> test has to be done anyway, but I would be grateful for any tips.
>
> Thanks,
> Michal Krompiec
> Merck KGaA
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GIUSEPPE MATTIOLI
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