[CP2K-user] CUTOFF and REL_CUTOFF

[Pierre-André Cazade]

Dear CP2K users,

Following the tutorial to tune the CUTOFF and the REL_CUTOFF for two of my 
systems, I find that quite large value of the CUTOFF  (700 Ry for beta 
glycine and 900Ry for quercetin) are necessary to achieve a decent level of 
precision (about 1.e-6). I am a bit surprised considering my systems are 
made from C, N, O, H. On top of that, for beta glycine, the last grid is 
not used that much, unlike what is recommended in the tutorial. Therefore, 
I have a few questions:

1/ Are these large cutoffs normal, even for small atoms? Is the cutoff more 
dependent on the crystal structure than the atom type?
2/ Is it normal to have still a large number of gaussians distributed on 
the first grid? Should I always aim for having a larger number of gaussians 
on the coarsest grid than on the thinnest?
3/ How do one decide on the number of grids? Is 4 kind of a magic number or 
should one aim to tune this as well? How does it affect 
accuracy/performance?

I attached the input file I used to generate the series of CUTOFF and 
REL_CUTOFF for both systems as well as the results I obtained.

Thank you for your help.

Best regards,
Pierre  
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