[CP2K-user] [CP2K:14828] Re: Conflict of results

[Monu Joy]

Hi Leo

Thanks for the help. I started a CASTEP calculation on the CP2K twisted
(relaxed) geometry and it converged as twisted, and this CASTEP relaxed
twisted geometry is more stable than the untwisted version by a factor of
0.00030737 eV. Gamma point was used for both twisted and untwisted
calculations.

Moreover, I have tried the CP2K calculation as you suggested to untwist the
geometry but it converged as twisted. I am using CP2K 7.1 and the only
change that I have made in the input was the removal of the following
lines:

        #COORD_FILE CP2K_Optimzed structure_CIF.cif
        #COORD_FILE_FORMAT CIF

Are there any significant changes between 7.1 and 8.1 in terms of dealing
with symmetry...?

-Monu

On Wed, Feb 24, 2021 at 3:50 AM Leopold Talirz <leopol... at gmail.com>
wrote:

> Hi Monu,
>
> thanks for following up - I did run a CP2K cell optimization starting from
> your relaxed "twisted" CP2K geometry. You find the trajectory and movie
> attached
> Does this agree with your results?
>
> It certainly seems to "untwist" the ligands (see movie) - I had to stop
> the optimization at some point though because convergence became very slow.
>
> Some stats from the total energy:
>
> First total energy (single-point): -2160.041714658635101
> First optimization step: -2160.136731180092738
> 50th optimization step: -2160.2817551900
> 100th optimization step: -2160.2996248085
>
> I.e. after the first "untwisting" that lowers total energy by  ~260mHa
> (quite significant), the final 50 steps lower the total energy by less than
> 3 mHa.
> This is with cp2k 8.1 [2] and with minimal modifications to the input file
> (see tar file). It's not clear to me why you would get a different result -
> both MULTIPLE_UNIT_CELL 1 1 2 and 2 2 2 K-points should give this result.
>
> As to why the CASTEP gamma point calculation does not yield the twisted
> structure: it is possible that this involves breaking a symmetry that is
> for some reason preserved in the CASTEP calculation (don't know).
> Can you try starting your CASTEP calculation from the twisted CP2K
> geometry and see where it ends up?
> If it remains twisted, you can then compare total energies to the
> untwisted result, which should tell you which one is the actual minimum.
>
> Best,
> Leo
>
> [1] Try  "grep i 925-pos-1.xyz" on the trajectory
> [2] Our previous calculations were with cp2k v5.1 though.
>
> On Tuesday, 23 February 2021 at 05:45:09 UTC+1 Monu Joy wrote:
>
>> 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 <
>> leo... 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
>>
>>
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