[CP2K-user] [CP2K:18127] Minimum supercell size for periodic calculations

[Eric Patterson]

Hello Xavier,

It was actually your paper that gave me the idea that something like 16 molecules and/or ~10 Angstroms per side cell might be a good general guideline. I appear to be getting sufficient results with this approach, but your caution about CL20 is instructive.

Thank you!

Best,
Eric


> On Dec 1, 2022, at 12:43 AM, Xavier Bidault <jazzquark at gmail.com> wrote:
> 
> 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 <mailto: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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