[CP2K-user] [CP2K:15075] OT/Supercell vs. Kpoints: Abnormal pressure difference

[Nicholas Winner]

Hi Lucas, good points you made.

The biggest thing that surprised me was not that 250 gamma point was not 
equivalent to 13x13x13 kpoint calculation on a primitive cell. I expected 
some difference. The biggest surprise was that the OT calculation converged 
to something very similar to Materials Project, while the kpoint 
calculation converged to a complete different polymorph. The Materials 
Project structure was calculated with kpoint grid and high plane wave basis 
cutoff, which confirmed Fm-3m is the ground state polymorph in agreement 
with experiments. While Materials Project's VASP calculations should not be 
equivalent to CP2K, with high quality basis/cutoffs/and the same kpoint 
grid, the results should be similar, as they both use good 
pseudopotentials, but it wasn't.

That's why I thought there had to be something off about how I was using 
the kpoints in CP2K. 

Also I just forgot to turn UKS off. Probably wasted some core-hours, but 
won't make the results worse.

Any further thoughts?

-Nick
On Friday, April 2, 2021 at 5:00:16 PM UTC-7 Lucas Lodeiro wrote:

> Hi Nicholas,
>
> I am seeing some definitions which are not totally comparable. For 
> example, the primitive cell of Si has 2 atoms, and if you use a 13x13x13 
> kpoint grid, the comparable supercell using just the gamma point is a 
> 13x13x13 supercell containing 4394 atoms. As you mention, for gamma point + 
> supercell calculation, you use a cell with 250 atoms (5x5x5 supercell) 
> which is comparable with a primitive cell calculation with 5x5x5 kpoint 
> grid.
>
> Abot the difference using different basis sets, well is not weird, the 
> stress tensor calculation is highly dependent on the scf convergence and 
> the accuracy of the wavefunction. 
>
> Another thing that caught my attention was the OT vs diagonalization 
> comparison. Both probably give a pretty similar solution, but no equal... I 
> would f¿prefer compare the primitive and supercell calculation using the 
> same wavefunction solver, especially if you use smearing, which is not 
> implemented for OT method, and could be a source of discrepancies (albeit 
> in a semiconductor this is not so important, if you are using a GGA or LDA 
> functional, the band gap is highly underestimated and the smearing affects 
> the calculation).
>
> Why are you using UKS instead of RKS?
>
> Regards - Lucas Lodeiro
>
> El vie, 2 abr 2021 a las 15:14, Nicholas Winner (<n... at berkeley.edu>) 
> escribió:
>
>> Hello all,
>>
>> I performed two cell optimizations on bulk Si, the first with a primitive 
>> cell and kpoints and the second using OT and a 250 atom supercell. Both 
>> structures started from the Materials Project (mp-149) lowest energy Fm-3m 
>> polymorph.
>>
>> I found that while OT only took 3 cell opt steps, the kpoint calculation 
>> took 10 steps, which I found a little strange.
>>
>> Upon investigation I found two things: 
>> (a) the kpoint calculation, had a large initial pressure deviation of 
>> -92570 bar, compared to OT, which had an initial deviation of  -708 bar at 
>> DZVP level and -2934 at TZV2PX level. Again these are the same structure, 
>> just kpoints vs supercell. It is a little odd to me that DZVP is closer to 
>> converged pressure, but I don't think its too strange.
>> (b) the kpoint calculation relaxed from Fm-3m to Im-3m space group, which 
>> is not the ground state polymorph.
>>
>> My question is whether or not this issue is due to CP2K's kpoint 
>> implementation, or if this is something more fundamental, and how (if at 
>> all) it can be resolved. Below is the abridged DFT section for the kpoint 
>> calculation, which I assume is where the problem lies, but I can provide 
>> more info if needed.
>>
>> Best,
>> Nick
>>
>> &DFT
>>
>>                 BASIS_SET_FILE_NAME BASIS_MOLOPT
>>
>>                 BASIS_SET_FILE_NAME BASIS_MOLOPT_UCL
>>
>>                 POTENTIAL_FILE_NAME GTH_POTENTIALS
>>
>>                 UKS True
>>
>>                 MULTIPLICITY 0
>>
>>                 CHARGE 0
>>
>>                 &QS
>>
>>                         METHOD GPW
>>
>>                         EPS_DEFAULT 1e-12
>>
>>                         EXTRAPOLATION PS
>>
>>                 &END QS
>>
>>                 &SCF
>>
>>                         MAX_SCF 100
>>
>>                         EPS_SCF 1e-07
>>
>>                         SCF_GUESS RESTART
>>
>>                         MAX_ITER_LUMO 400
>>
>>                         ADDED_MOS 100
>>
>>                         &DIAGONALIZATION
>>
>>                                 &DAVIDSON
>>
>>                                         PRECONDITIONER FULL_ALL
>>
>>                                 &END DAVIDSON
>>
>>                         &END DIAGONALIZATION
>>
>>                         &MIXING
>>
>>                                 METHOD BROYDEN_MIXING
>>
>>                                 ALPHA 0.2
>>
>>                                 NBUFFER 5
>>
>>                         &END MIXING
>>
>>                         &SMEAR
>>
>>                                 ELEC_TEMP 300
>>
>>                                 METHOD FERMI_DIRAC
>>
>>                                 FIXED_MAGNETIC_MOMENT -100.0
>>
>>                         &END SMEAR
>>
>>                 &END SCF
>>
>>                 &KPOINTS
>>
>>                         SCHEME MONKHORST-PACK 13 13 13
>>
>>                         EPS_GEO 1e-06
>>
>>                         FULL_GRID False
>>
>>                         PARALLEL_GROUP_SIZE -1
>>
>>                         SYMMETRY False
>>
>>                         UNITS B_VECTOR
>>
>>                         VERBOSE True
>>
>>                         WAVEFUNCTIONS COMPLEX
>>
>>                 &END KPOINTS
>>
>> &END DFT
>>
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>>
>
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