[CP2K-user] [CP2K:14800] Re: Conflict of results

[Monu Joy]

Even though the doubling of the z-axis (MULTIPLE_UNIT_CELL 1 1 2) seemed fine in the initial cycles, it ended up being twisted on convergence. Then I have tried the K-point scheme (SCHEME MONKHORST-PACK 2 2 2), where the result was the same, twisted. However, the structure seems reasonable (no twisting) in CASTEP even with the gamma point.
But of course, the culprit could be the small z-axis, but I am not sure how to tackle this in CP2K.
-Monu 

    On Friday, 19 February, 2021, 05:25:24 pm GMT-5, Leopold Talirz <leopol... at gmail.com> wrote:  
 
 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 

-- 
You received this message because you are subscribed to the Google Groups "cp2k" group.
To unsubscribe from this group and stop receiving emails from it, send an email to cp... at googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/cp2k/a018da49-5c42-4ade-b9f5-1c1b68579a91n%40googlegroups.com.
  
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.cp2k.org/archives/cp2k-user/attachments/20210223/5ae65a7c/attachment.htm>