[CP2K-user] [CP2K:14513] Re: Conflict of results

[Leopold Talirz]

Since Andres Ortega from our group noticed this with a similar Al MOF, I 
just wanted to confirm:

Thanks, Marcella, for the suggestions. I just increased the Z direction by MULTIPLE_UNIT_CELL  
> 1 1 2. Albeit it's not yet converged, the intermediate geometries are 
> reasonable (no twisting so far). 
>
This is indeed the source of the issue here - doubling the cell along z 
results in a reversal of the twisting of the ligands, even when starting 
from the twisted geometry (I checked).

In our case, the twisted structure was the correct minimum for the 
unit-cell only calculation, with a substantially lower energy (~250meV per 
twisted phenyl), while for the doubled cell the twisted structure was 
slightly higher in energy (order of 10meV per twisted phenyl).

This then re-raises the original question of the thread, which is why 
CASTEP seems to yield a different result. I believe the answer is in the 
CASTEP output:

                           -------------------------------
                              k-Points For BZ Sampling
                           -------------------------------
                       MP grid size for SCF calculation is  1  1  2
                            with an offset of   0.000  0.000  0.000
                       Number of kpoints used =             1

CASTEP chooses a 2-kpoint grid along z (which reduced to just a single 
k-point calculation through symmetry).

>From the CASTEP manual [1]:

> For cubic cells and for the C direction of hexagonal cells the even and 
odd grids for the Monkhorst-Pack scheme give the same number of k-points. 
However, the even grid provides better sampling and will always be used 
automatically under these conditions. This ensures that a good grid with 
k-point separation at (or less than) the specified target can be achieved 
more economically. As a consequence, such lattices may have much finer 
separations than requested as odd grids have been excluded - even though 
they would have been closer to the specified separation - and the better 
even grids have been taken in preference.

I suspect that forcing CASTEP to do a Gamma-point calculation would result 
in the same twisting as seen in CP2K (in our case, we checked with CRYSTAL).
For our MOF, just to exclude any possibility of issues with OT, we also 
tested this with DIAGONALIZATION and KPOINTS in cp2k and got the same 
result as with the 1x1x2 supercell.

The lesson is simply that ~6.5A can be too short along the direction of 
these inorganic chains/rods in MOFs.

Best,
Leopold

[1] http://www.tcm.phy.cam.ac.uk/castep/documentation/WebHelp/CASTEP.html#modules/castep/dlgcastepelecoptkpoints.htm?Highlight=monkhorst
 
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