[DFTB-Plus-User] question about UFF dispersion

[Bálint Aradi]

Dear Nir,

> When using UFF dispersion with a periodic lattice, it appears that v1.2
> of the code removes the 1/r^6 term from the Lennard-Jones part of the
> potential (e.g., when rr > r0) and moves it to the polynomial part  (rr
> < r0). Why is that? This seems to contradict what’s shown in the 2005
> JCTC Zhechkov paper.

I hope, we do not. We use Ewald-summation technique in order to sum up
the 1/r^6 terms. In order to do that, in periodic systems we subtract
the 1/r^6 term in the real-space calculation (from the Lennard-Jones
part and also from the polynomial part) and then add it vie Ewald summation.

I did a short test for an C2 dimer at various distances. I obtain almost
exactly the same dispersion energy profile, whether I calculate it as a
cluster or as a molecule in a large box. The code-path in DFTB+ is,
however, different.

  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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