Possible bug in DFT-D3

[Leopold Talirz]

Dear all,


a colleague in my lab found an unexpected dependence of the DFT-D3 
dispersion energy on the amount of vacuum in a slab calculation, which 
could indicate a bug in the DFT-D3 implementation.


Here the message from Sasha:

I have a slab geometry for a slab that is ~20 A thick (less than 20) and I 
perforrm a fully periodic calculation.

To test the convergence of the vacuum region i performed ENERGY 
calculations with a cell of 40 A  (so 20 A of vacuum) and, afterwards, the 
same ENERGY calculation with a cell thickness of 60A (so 40 A of vacuum).


I did define R_CUTOFF for the VDW as 8 A, i.e. less than half of the vacuum 
region. I was therefore expecting the dispersion energy to be exactly the 
same (?).


The ENERGY calculations for the SAME geometry (file p.xyz) give rise to a 
difference of 1.8eV in the dispersion energy term for the DFT-D3 
calculation, when i change the cell size.

I repeated the same calculations with DFT-D2 and in this case there is no 
difference in the dispersion energy (as I would have expected).


In the tar file you can find the input that I used for the four different 
calculations (DFT-D3 with 20 and 40 A of vacuum  and DFT-D2 same two cases) 
as well as output files and the geometry p.xyz.

Does anybody have an idea on what could be wrong?


Thanks in advance for your help


Kind regards,

Sasha (and Leopold)


P.S. I used cp2k revision svn:14377
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