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

[Omer Mutasim]

Dear Moraga Your feedback is very helpful 👌. I have another questions :Is it fine to use the default value (1xe-03) for force-conv-thr ? If i use this default value , what is the suitable value conv-thr ? I’m familiar with this asymmetric slab, so How to use the dipole background You stated?In order to study various monolayer coverages , i have yo place multiple S atoms on the surface, ao is there any criteria for placing these atoms on asymmetric slab ? How far they should be from each other to avoid lateral interactions between Adsorbed atoms ? 
Sulfur (S⁻²) adsorption, should i set the net charge of the system to -1 ? If i set it to 0 (default) , does that mean Sulfur will be bonded to two Ni atoms ? 
The Ni2P is paramagnetic, so I didn’t consider spin polarized calculation, is this correct ? 


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On Sunday, August 16, 2020, 11:19 PM, Lucas Nicolás Lodeiro Moraga <lucas.lodeiro at ug.uchile.cl> wrote:

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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