[CP2K-user] [CP2K:16303] van der Waals cutoff vs. system size

[Binquan Luan]

Dear all,
For DFT calculations with van der Waals corrections, I am wondering what 
the right cutoff distance should be. In the manual, I found

R_CUTOFF {Real} 
*Range of potential. The cutoff will be 2 times this value*  [Edit on GitHub 
<https://github.com/cp2k/cp2k/blob/master/src/input_cp2k_xc.F#L875>] 
This keyword cannot be repeated and it expects precisely one real. 
Default value: 1.05835442E+001 
Default unit: <https://manual.cp2k.org/trunk/units.html> [angstrom]

If the default range of potential is 21 angstrom, with the minimum image 
convention used for PBC, the system size (in each dimension) should be at 
least 42 angstrom that is much larger than most systems reported in 
literature.  Do I miss anything here? 

Thanks a lot for help!

Binquan

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