[CP2K:5706] Possible bug in DFT-D3

[Carlo Antonio Pignedoli]

Dear Flo,
I really fear it is a D3 problem, 
we would have not done the D2 test otherwise.
We are very familiar with cutoff problems, that’s why here the cutoff is 600 Ry (despite 800 would be better for Pd)

we are speaking here of 0.07 Ry difference just in dispersion energy..

Unless, as Leo said, there is something non trivial that we ignore 
i would guess that something with R_CUTOFF is wrong but I cannot imagine what

Kind regards

Carlo

On 06 Oct 2014, at 22:25, Leopold Talirz <leopold... at gmail.com> wrote:

> Dear Flo,
> 
> thanks a lot for your reply and the link. Nice table with the cutoff values for the elements by the way :)
> 
> Concerning our question - I completely agree with your point, where it concerns quantities that are computed on the grids. Our question, however, concerns only the *dispersion energy*. As far as I know (please correct me, if I am wrong) both DFT-D2 and DFT-D3 compute the dispersion energy based only on atomic coordinates. 
> If the interaction between the periodic replicas of the slab across the vacuum region are set to zero by R_CUTOFF, then the dispersion energy should be completely independent of the amount of vacuum. If you check the values for the DFT-D2, you will find that the dispersion energies for the two different amounts of vacuum are identical up to the last digit.
> 
> Best,
> Leo
> 
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