[DFTB-Plus-User] Energy difference between DFTB3 or SCC-DFTB with HF or CCSD(T)

[Bálint Aradi]

Dear Prasanta,

your are missing the fact, that different methods attribute different 
energies to the isolated atoms, therefore, the reference energy in the 
calculations is different. More comparable are formation and 
dissociation energies, e.g.

E = E_tot - sum_i mu_i n_i

where the sum runs over the atom types in the system, n_i is the number 
of atoms of a given type in the system and mu_i is either the energy of 
the isolated atom (dissociation energy) or the atom in its standard 
state (formation energy). In energy differences between different 
configurations of the same system the 2nd term cancels, so those can be 
also compared between methods.

Best regards,

Bálint


On 14.09.21 11:11, prasantb wrote:
> Hi community members,
> 
> I have just ran my first few test calculations with DFTB+. I have found 
> a page for some test calculations, 
> https://dftbplus-recipes.readthedocs.io/en/latest/basics/firstcalc.html#band-out 
> <https://dftbplus-recipes.readthedocs.io/en/latest/basics/firstcalc.html#band-out>. 
>  From there I ran water molecule optimization. I ran that and found that 
> the Total energy is -4.0618820527 H. However, the HF or CCSD(T) 
> calculation shows the total energy to be ~-76 Hartree. I know that DFTB 
> can not give same energy with HF but this is a rather large deviation.
> 
> What is the problem or what am I missing here?
> 
> -- 
> Thanking You,
> Prasanta Bandyopadhyay
> 
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> 


-- 
Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen
http://www.bccms.uni-bremen.de/cms/people/b-aradi/


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