[CP2K-user] [CP2K:20427] UKS + BCC Iron ( setup and convergence )

[Ji Woong Yu]

Dear CP2K users,

I am calculating the energy of a slab (periodic in XY and non-periodic in 
Z) using CP2K. I have included my original CP2K input file for reference.

The problem is that I cannot achieve convergence with UKS. Perhaps it's due 
to my impatience, but the convergence cannot go below 0.01 after a few 
hundred iterations.

SCAN (including R2SCAN) is known for its difficulty in converging. However, 
for silicon, it converges well with the same settings, though it doesn't 
require UKS (this was just for experimentation).

After changing the POISSON_SOLVER to IMPLICIT, I observed a slight 
improvement in the convergence profile. However, it still did not fully 
converge.

Upon adding different options like the ones below, I saw a significant 
improvement

 &POISSON
   PERIODIC XY
   POISSON_SOLVER IMPLICIT
   &IMPLICIT
    BOUNDARY_CONDITIONS NEUMANN
    NEUMANN_DIRECTIONS Z
   &END IMPLICIT
  &END POISSON

However, there is a discrepancy between the energy scale where the original 
script struggled (around -17794 Hartree) and the one shown by this 
modification (-18097 Hartree). The simulation is still running but seems 
promising, at least in terms of convergence trend through iterations.
My questions are:

(1) Are the boundary conditions shown above appropriate for solid metal 
slab calculations? The energy discrepancy induced by adding options makes 
me suspicious of using an improper method. Or could it be that we've 
finally found the correct energy level?
(2) Does my original code have an intrinsic problem that prevents 
convergence, or is it simply a matter of needing more patience to allow it 
to converge?

Thanks in advance,

Ji Woong

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