[DFTB-Plus-User] Help Bad results in testing binding energy and reaction energy of small molecules with DFTB+

[Bálint Aradi]

Dear Lin Lai,

> In order to find out whether my DFTB calculations are correct, I try to
> reproduce some results about reaction energy using DFTB+ code (I couldn't
> find any materials evaluating the binding energy calculated with SCC-DFTB,
> do I miss something?). I refer the article PRB 58, 11, 7260 (1998), where
> the reaction energy for the reaction (NH2NH2+H2-------> 2NH3) is about 30
> kcal/mol (SCC-DFTB), 43 kcal/mol (LSD), and 48 (experiment) respectively,
> and the reaction energy for another reaction (N2+3H2------->2NH3) is 37
> kcal/mol (SCC-DFTB), 71 kcal/mol (LSD), and 37 kcal/mol (experiment). From
> my own calculations, the total energy for NH2NH2, NH3, N2, and H2 molecules
> are -171,50 eV, -96.55 eV, -133.19 eV, and -18.37 eV, respectively. So the
> obtained reaction energies for above two reactions are  3.23 eV (74.51
> kcal/mol) and 1.96 eV (45.20 kcal/mol), respectively. In my opinion, these
> results are too bad and I doubt my calculations are incorrect.

 Please note, that the mentioned paper uses the mio-set. This is the
one, you should use for the calculations you describe above. In case,
you can not reproduce the results, you should try to contact the authors
directly.

  As for the binding energies: One should be rather careful with the
energy of the atom in DFTB+, as it is not really well defined (It is the
sum of the valence eigenvalues times the occupation). One should -- in
my oppinion -- rather compare energies of bonded systems (reaction
energies).

  Best regards,

    Bálint


-- 
Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen
http://www.bccms.uni-bremen.de/en/~b_aradi


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