<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;">Hello Xavier,<div><br></div><div>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.</div><div><br></div><div>Thank you!</div><div><br></div><div>Best,</div><div>Eric</div><div><br><div><br><blockquote type="cite"><div>On Dec 1, 2022, at 12:43 AM, Xavier Bidault <jazzquark@gmail.com> wrote:</div><br class="Apple-interchange-newline"><div><div dir="auto">Hi Eric,<div dir="auto"><br></div><div dir="auto">You can have a look at these papers and relevant details in their supplementary information.</div><div dir="auto"><a href="https://doi.org/10.1063/5.0041511">https://doi.org/10.1063/5.0041511</a><br></div><div dir="auto"><a href="https://doi.org/10.1039/D2RA05062E">https://doi.org/10.1039/D2RA05062E</a></div><div dir="auto"><br></div><div dir="auto">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.</div><div dir="auto"><br></div><div dir="auto">You can also see my concerns about Grimme D2 vs D3(BJ). But this may only concern nitramines.</div><div dir="auto"><br></div><div dir="auto">Yes, NLCC GTH PP with Molopt-DZVP in CP2K should do the job. But no less then 600 Ry for the energy cutoff.</div><div dir="auto"><br></div><div dir="auto">Best,</div><div dir="auto">Xavier</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">Le mer. 30 nov. 2022, 5:16 PM, Eric Patterson <<a href="mailto:eric.v.patterson@gmail.com">eric.v.patterson@gmail.com</a>> a écrit :<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi all,<div><br></div><div>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? </div><div><br></div><div>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?</div><div><br></div><div>I plan to use PBE with GPW, MOLOPT basis sets, and NLCC potentials if that matters.</div><div><br></div><div>Any guidance is appreciated.</div><div><br></div><div>Thanks,</div><div>Eric</div><div><br></div><div><br class="webkit-block-placeholder"></div>
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