[CP2K-user] [CP2K:22222] Extrapolation formula for basis set convergence

[Riccardo Pezzetta]

Dear all,
I am doing a calculation for the binding energy of the argon dimer with RPA 
and I used as primary basis sets aug-cc-pVTZ and aug-cc-pVQZ. In the CP2K 
maual I saw that I have to use the extrapolation formula E(X)=A+B/X^3 for 
correlation energies, but I don't know how to extrapolate the EXX energy. 
In the article https://pubs.acs.org/journal/jctcce I saw that they use the 
extrapolation formula for the cohesive energies instead of correlation 
energies. So I tried to make the extrapolatoin in two ways:
1) I calculated the binding energy of the dimer with TZ and QZ basis sets 
separately and made the cubic extrapolation with the resulting bonding 
energies
2) I made the extrapolation with the cubic formula with absolute RPA and 
EXX energies for the dimer and the atom separately and after that I 
calculated the bonding energy with the extrapolated results.
With the first method I get E bonding = -8.4 meV, with the second one I get 
E bonding = -8.6 meV.
Are these procedures correct? Since the results are similar can I deduce 
that both methods are correct? In both cases the atom calculation has been 
made with a ghost atom with the same geometry of the dimer in order to take 
into account the basis set superposition error.
Kind regards;

Riccardo Pezzetta

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