[Wien] Strategy for a large slab with SP and SOC

[pluto]

Dear Prof. Blaha, dear All,

Thank you for the comment on slab strategy, this helps a lot.

I have more specific question: for a large WTe2 slab (60 atoms), which 
is a material of low-symmetry that has a polarity also in the 
out-of-plane direction, I am getting ghostbands in lapw2 after few 
iterations. What is a good strategy to fix this?

I was thinking of:

1. init_lapw -hdlo
2. Low mixing (like 0.05) with PRATT
3. Decrease RMT (from first tests, with RMT 2.5 ghostbands seem to 
appear after around 3 iterations, with 2.2 after many iterations)
4. Increase RKmax

3 and 4 are probably computationally expensive...

I did several tests without SOC, I was typically using something like:

init_lapw -sp -b -numk 100 -hdlo -fermit 0.002

Maybe other settings are critical?

Bulk calculation converges very easily (first without and then with SOC) 
with default settings like

init_lapw -sp -b -numk 2000

bulk bands look like the literature, and are practically the same with 
RMT 2.2 and RMT 2.5.

Best,
Lukasz




On 2023-06-16 16:45, Peter Blaha wrote:
> No,this is not a good strategy.
> 
> From a converged non-spin-polarized calculation you cannot come
> (easily) to a spin-polarized solution.
> 
> So   1) is only good if you want to quote how much more stable a SP
> solution is compared to a non-SP.
> 
> 2 + 3 is a good practice. You gain insight how large are the changes
> and on what atoms due to SO coupling.
> 
> ------------------------
> 
> In terms of efficiency for large cases, I'd in particular preconverge
> with a course k-mesh and later on refine.
> 
> ---------------------------
> 
> Every runsp cycle starts with a case.clmsum/up/dn file.
> 
> These files can come from an initialization, but of course also from
> any prior scf calculation (eg. with lower k-mesh or without SO). Of
> course, a restore_lapw ... gives you all files necessary to run
> another scf cycle.
> 
> -NI would keep old broyden files, but after a "save_lapw" they are
> gone anyway.  -NI is useful if you want to continue a scf, because eg.
> the first runsp stopped after 40 cycles and did not reach convergence
> yet.
> 
> Am 16.06.2023 um 10:44 schrieb pluto via Wien:
>> Dear All,
>> 
>> I just would like to confirm the step-by-step convergence strategy for 
>> the large slab with SP and SOC (it refers in general to spin-momentum 
>> locked non-magnetic TMDC, but can be any other material).
>> 
>> Is the following correct:
>> 
>> 1. Converge without SP and without SOC, and save_lapw e.g. as 
>> CONV_NO_SP_NO_SOC so it can be used in another directory or on another 
>> computer for the next steps
>> 2. Use this as a starting point to converge with SP, and save_lapw as 
>> CONV_W_SP_NO_SOC (one can also restore_lapw in another directory and 
>> start there)
>> 3. Use this as a starting point to converge with SP and with SOC (and 
>> save_lapw to have it for the future)
>> 
>> I often start with step 3 right away, but I think for a really large 
>> system this might be really inefficient.
>> 
>> How does the program know to use the starting density from the 
>> previous step?
>> Does restore_lapw creates the necessary files when I transfer to the 
>> new directory?
>> Is -NI or some other setting in run_lapw important here?
>> 
>> At the moment I am using an older cluster with many cores and use 
>> k-parallel. Still didn't manage with MPI, but maybe it is not needed 
>> for what I want because my klist file is typically 50-80 k-points, 
>> depending on the symmetry of the system. I use the QTL program quite a 
>> lot so having it parallellized would sometimes speed the things up a 
>> bit for me.
>> 
>> Best,
>> Lukasz
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