[Pw_forum] converging V_xc in the vacuum

[Janos Daru]

Dear experts,

I would like to print out plane averaged exchange-correlation potential for
a PBE Cu(111) slab along its normal vector (z-direction).
To this end I have printed out plot_num:

1  = total potential V_bare + V_H + V_xc
and
11 = the V_bare + V_H potential

and subtracted the second from the first and averaged it.
However, unlike in JCTC 2009,5,881-886 (figure 3.) or in Nano Lett.
2015,2448-2455 (figure 3.) V_xc does not converge to zero in the vacuum
region (it has an approximate value of  -0.015 Hartree).

The SI from the second paper authors explains that:
"we note that it is well-known that in the vacuum the exchange-correlation
(xc) part converges much more slowly than the electrostatic part of the
total potential. Therefore, when computing the local potential of bare
graphite(0001) and Au(111), we employed more strict settings in the
band-structure calculations. Specifically, we increased the number of
FFT-grid points and enhanced the representation of the charge density,
potential and augmentation charges. Furthermore, we have employed an
additional support grid for the augmentation charges, which further
increases the accuracy of calculated quantities."

So I tried to tighten ecutrho, the realspace grid, the convergence
criteria, I have increased the vacuum, number of Cu layers etc. but V_xc
does not converge to zero in the vacuum. Could some of the experts explain
to me if it is due to a wrong setup, or eg. due to a different reference
level?

I have attached the full input, and V_xc plot. I have used QE 5.2.0 for my
calculations.

Thank you for the answer in anticipation,

Janos Daru
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