[CP2K-user] Cell optimization of layered material, convergence vs K_POINTS

[Daniele Ongari]

Dear CP2K developers, 

while optimizing the cell of 2d layered covalent-organic frameworks (think: 
graphite with holes) we are encountering instabilities that can seemingly 
be quenched by increasing the number of layers used in the cell (increasing 
the K_POINTS works as well).

Since the materials tend to have a PBE gap above 2eV and should be 
van-der-Waals bonded only, we find that surprising.


In the following we report one of these problematic cases, COF-1, using 
PBE-D3(BJ) functional.

Figure 1 shows the starting unit cell, which contains 2 layers of the 
material (before and after the cell_opt with kpoints).



Figure 2 shows the convergence of the total energy, once using just the 
Gamma point and once using 1x1x2 k-points (2 k-points along the z 
direction). Note that for the second one we use DIAGONALIZATION and 
EXTRAPOLATION=USE_GUESS, as required for using kpoints, but the same 
CELL_OPT settings.


The cell find its optimal dimension by tilting in the xy plane and, in the 
case of Gamma calculation, the optimization oscillates without converging.

- ot-gamma inp: https://www.dropbox.com/s/g5usnojtblhomi3/ot_inp.txt?dl=0

- kpoints inp: https://www.dropbox.com/s/d69170jnqfuq4g6/kp_inp.txt?dl=0

- coord+inputs+outputs zipped: 
https://www.dropbox.com/s/k3kl6sw9u8ine2y/cof1_ot_vs_kpoints.zip?dl=1


We used K-POINTS because the perpendicular width in z direction is 6.920 
Angs at the start and gets 6.766 Angsat the end, however we are confused on 
why the K-POINTS in z directions are so important for the stability of the 
calculation. Is there any other parameter that I can tune to stabilize the 
no-K-POINTS+OT calculation?

I tested already many options without kpoints, without success (always 
experiencing jumps in the energy during the CELL_OPT):

1) lower EPS_SCF
2) higher CUTOFF
3) DIAGONALIZATION
4) EXTRAPOLATION=USE_GUESS

and with (a partial) success:

5) Duplicating the cell in z direction gives almost exactly the same result 
as with K-POINTS (and is more efficients in terms of speed, albeit not an 
elegant solution)
6) CG or (much) lower TRUST_RADIUS=0.1, do not jump but they are *very* 
inefficient, and therefore not a solution.
7) Using the PW code Quantum Espresso (much slower than CP2K for this 
system) with Gamma approximation. In that case PBE-D3 does always lead to a 
smooth cell_opt convergence using bfgs. As expected, the z dimension of the 
cell is small, but "not so small" to cause problems.

Therefore, the most efficient solution for now it would be to duplicate my 
2D materials in the z direction (mod #5) to have a robust convergence with 
the price of a ~2x slower calculation and having a bigger reference cell. 
This does not *always* solve the cell_opt convergence problems, but it 
works for most of the problematic cases. Consider that, as in my previous 
post on OT convergence problems (
https://groups.google.com/forum/#!msg/cp2k/X8RhYjSVVWc/TXdeLTMUCQAJ) our 
aim is to use general setting for high-throughput calculation. 

Do you have any other suggestion to try solving my problem?
I expect that these 2D-layers have a corrugated potential surface: I'm not 
looking for the absolute minima, but at least a reasonable one without 
geometry convergence problem. I'm wondering if there are some threshold 
value that I can tune not to have interference with the CELL_OPT, without 
increasing much the computational cost (not more than ~2x).

Any suggestion is welcome, thanks!

Daniele Ongari

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