[Q-e-developers] Vacuum potential for Platinum slab using 'vdW-DF-obk8' functional

[Aldo Ugolotti]

Dear developers,
I'm working on an adsorbate system based on a Pt slab. Since I need to
account for Van der Waals interactions I chose a functional of the vdw-df1
family, specifying "input_dft = 'vdW-DF-obk8' " in input files. I found a
strange behaviour of the code using the pseudopotential file
"Pt.pbe-nd-rrkjus.UPF", which gives a potential well in the vacuum region,
as shown in picture 'Pt111.1x1.pbeold.eps'. It allows a small fraction of
the charge density to slip there from the surface, resulting in a small but
not negligible LDOS. As a test I downloaded a newer PP from QE library
"Pt.pbe-n-rrkjus_psl.0.1.UPF" and tried it. As can be seen in picture
'Pt111.1x1.pbenew.eps' the same feature is absent. As last test I tried the
first PP changing the cutoffs and the k mesh size: the results are shown in
picture 'pbeold-optb88vdw-convergency.eps'. I got the same results using QE
v 5.1.2 and 5.2.0.
I'm also attaching a complete script that can be run for checking and
testing. It's based on QE examples, so it could be run from any computer.
I'd like to ask you if this is a general issue, so even using the 2nd PP
the results are not reliable or is it just a problem of the first one?

Thanks in advance,

Regards

Aldo Ugolotti
Master thesist at Università Milano-Bicocca
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