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

[Eric Patterson]

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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