[QE-users] Subject: RE: Slab doesn't converge

[Lucas Nicolás Lodeiro Moraga]

Hi!

I am not an expert in adsorption, but I thought your input has some
deficiencies.

Firstly, forc_conv_thr is set for the maximum force for each relax-frame,
the value that you use is too low, compared with your conv_thr (which
define how accurate are your scf forces). Also, for relax, stress is not
necessary and you could save some time neglecting it (I assume your slab is
converged after for vc-relax).

About the electronic parameters... 25 Ry for ecutwfc seems to me too low
for USPPs. The fermi-dirac smearing is slower than others and the results
do not change appreciably. Another thing is the vdW approach, you are using
a vdW-functional plus the grimme's vdW-corrections... this does not seem
correct. If you use a vdW-functional you are accounting the vdW
interactions by the non-local correlation term, and the grimme's empirical
correction is not needed, and vice versa, if you use the grimme's empirical
corrections, you have to use a non vdw-functional as LDA,GGA or hybrid.

A value of 0.7 for mixing_beta could be problematic for bad behaved
systems. When I have problems on the first scf, I change this value to
lower values like 0.5 or 0.3 to prevent long jumps in the scf solutions.

Finally, something about the system-structure is weird to me. Your
distances between S-P and S-Ni are too short, ~1.5 A. Those distances are
too short to heavier atoms, and seems to me a bad guess for the first
geometry (but I remember, I am not an expert in adsorptions, you know more
about the system).
Moreover, as you are putting the adsorbate only on one face of the system,
you have a net dipole in the system. If you print the electrostatic
potential profile along z, on vacuum space you will have a slope on the
profile. It can be fixed, using a dipole background or using symmetrical
adsorbates and surfaces on both sides of the slab.

Also, you are simulating the adsorption of neutral atoms (in principle) of
S. If you want to simulate Sulfur (S⁻²) adsorption, your system has to be a
total negative charge, this requires some 2-D method to neglect the
interactions between both sides of slab.

Regards
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