[CP2K-user] [CP2K:18125] Minimum supercell size for periodic calculations

[Xavier Bidault]

Hi Eric,

You can have a look at these papers and relevant details in their
supplementary information.
https://doi.org/10.1063/5.0041511
https://doi.org/10.1039/D2RA05062E

As a matter of fact for epsilon-CL20, I could have just used a 2x1x1
supercell, making every dimension larger than 10 A, but an attempt with a
2x2x2 supercell showed that 2x1x1 was not enough (unconverged low frequency
range of the vDOS). My guess was because the crystal is not orthorhombic
and that I had to use a larger supercell to allow for more vibrational
freedom.

You can also see my concerns about Grimme D2 vs D3(BJ). But this may only
concern nitramines.

Yes, NLCC GTH PP with Molopt-DZVP in CP2K should do the job. But no less
then 600 Ry for the energy cutoff.

Best,
Xavier

Le mer. 30 nov. 2022, 5:16 PM, Eric Patterson <eric.v.patterson at gmail.com>
a écrit :

> Hi all,
>
> I am a complete beginner with CP2K and would like to perform CELL_OPT
> followed by VIBRATIONAL_ANALYSIS on periodic solid molecular systems
> (mostly organics). Are there general guidelines for how big a supercell
> must be for results to be considered reliable, either in terms of absolute
> dimensions or # of atoms/molecules?
>
> By way of examples, if I study glycine, there are only two molecules in
> the unit cell, which I assume is much too small. If I use a 2x2x2
> supercell, there are 16 molecules (160 atoms) and the cell dimensions are
> between 10 and 12 Angstroms in all dimensions. If I chose benzoic acid, the
> c vector of the unit cell is already greater than 20 Angstrom, so I am
> considering a 2x2x1 supercell (16 molecules or 240 atoms, cell dimensions
> of 10 or 11 Angstroms in a and b, 21 in c). Are these reasonable choices?
>
> I plan to use PBE with GPW, MOLOPT basis sets, and NLCC potentials if that
> matters.
>
> Any guidance is appreciated.
>
> Thanks,
> Eric
>
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