[CP2K-user] [CP2K:19362] Isolate dispersion contribution in vdW-DF2 calculations

[Michael Fischer]

Dear all,

I am studying host-guest systems (molecules in zeolites) with the 
rev-vdW-DF2 functional (X_B86_R, PW92 correlation, LMKLL dispersion) and I 
am looking at the possibility to calculate the dispersion contribution to 
the total interaction energy.

In the past, I have used Grimme-type dispersion correction, e.g., PBE-D3. 
Here, it was fairly trivial to calculate the dispersion contribution as:
E_disp = E_disp(host+guest) - E_disp(host) - E_disp(guest)
...where the E_disp terms on the right-hand side correspond to the energy 
values given in the line "Dispersion energy" in the CP2K output.

With rev-vdW-DF2, "Dispersion energy" is also printed. However, what is 
notable in the first place is that the dispersion energy is always 
positive. A subtraction as in the above equation gives results that look 
plausible at first, but a comparison to the total interaction energies 
shows that the E_disp contribution is sometimes more negative than E_tot, 
something which seems implausible.

Is there some way to isolate the dispersion contribution in calculations 
using non-local vdW-DF approximations?

Any suggestion will be highly appreciated!
Thank you and best regards,
Michael

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