[DFTB-Plus-User] Why the Borg-0-1 parameter set gives a negative repulsive potential value?

[Bálint Aradi]

Dear Yen,

> The reference "SCC-DFTB parameterization for boron and boranes. J. Chem.
> Theory Comput. 8, p. 1153" states that the parameter set is applicable to
> finite and perodic systems. I assume that this means it can be used for
> boron clusters. Also in dftb website they claim that this parameter set is
> fitted to molecular system so i think it should work even better for boron
> cluster. Anyhow, it shouldn't generate a negative value for "repulsive "
> energy. Forget to mention, my system is B_20 and the input structure is
> obtained from a global optimization program.

In ideal cases, the repulsive should be positive indeed. However, if you
look on the repulsive of Bohr as stored in the B-B SK-file (see attached
figure) you see, that it is negative in a wide region (Energies are
given in Ha, lengths in Bohr), so no wonder that you obtain negative
repulsive energies.

Now, whether a negative "repulsive" potential is always an evil thing or
not, is a different question. I am more for the pragmatic approach: The
repulsive potential just contains all terms which are not explicitely
calculated in the DFTB approach + all corrections compensating for the
errors we did in the explicitely calculated (but strongly approximated)
terms.

So, the advice is always the same: just compare the results for small
systems agains ab initio results. If they are fine and you get
qualitatively good results, scale up to bigger systems.

  Best regards,

  Bálint


-- 
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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