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

[Lucas Lodeiro]

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 (<nwi... 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
>
> --
> 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/b7c861a2-eb23-4fac-8a42-9846e53f6f24n%40googlegroups.com
> <https://groups.google.com/d/msgid/cp2k/b7c861a2-eb23-4fac-8a42-9846e53f6f24n%40googlegroups.com?utm_medium=email&utm_source=footer>
> .
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.cp2k.org/archives/cp2k-user/attachments/20210402/4e2720dc/attachment.htm>