[DFTB-Plus-User] Bug fix of dispersion correction for periodic systems in release 1.2

Bálint Aradi balint.aradi at bccms.uni-bremen.de
Wed May 22 11:03:32 CEST 2013

Dear Christoph Lechner,

> Did this affect both dispersion models? We have used the Slater-Kirkwood
> model for our calculations.
> Did this affect both the gradient and singlepoint calculations?
> I ask this because I redid some calculations, geometries obtained with
> version 1.1 yielded the same singlepoint energy with version 1.2. I also
> redid some geometry optimizations and overall the optimized system had a
> lower energy starting from the same input geometry.

 The Slater-Kirkwood dispersion calculation was not directly affected,
provided you did only single point calculations or inoic relaxations,
but no cell optimisation. The cell optimisation was not implemented
correctly for any of the dispersion model, and due to a bug in the code,
it did not stop, when user tried to optimize their cell with dispersion
switched on.

 Did your calculation involved lattice optimization? If yes, I am quite
sure, the difference comes from the buggy optimization algorithm (stress
tensor was not correct for your system). If not, could you send me one
self containing example (including sk-files), so that I can try it myself?

> Do you have any experience, what the overall error of the dispersion
> correction is with version 1.1, since I have to decide if these
> calculations should be redone with the new version.

Hard to say, it depends how much the lattice vectors depend on the
dispersion interaction. I'd guess, that for strongly bound systems (like
your carbon nanotubes), the effect shouldn't be significant.

  Best regards,


Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen

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