[DFTB-Plus-User] [Fwd: MD periodic problem]

Bálint Aradi balint.aradi at bccms.uni-bremen.de
Tue Nov 17 09:27:46 CET 2009

Dear Debasis,

> I just started using DFTB+. I am trying to do a MD calculation with 
> periodic option.  This is disordered system (liquid like).   I see some 
> of the atoms/molecules are going out from the box and the cartesian 
> coordinates it prints are not within the box.  I am under the impression 
> that if an tries to go out from the box, it will be replaced by another 
> (image) so that the total number of atoms will be unchanged in the box.
> I was wondering if anyone can help.  I have attached a sample input and 
> final atom coordinates.  Thanks
> Debasis

  Usually, every supercell code (DFTB+ is one, provided you use it for a
periodic system, like in your case), models the system you provide with
periodic boundary conditions, meaning that atoms leaving the box on one
side, are folded back to the unit cell. It happens even if you don't see
it on the coordinates, because it is assumed, that every supercell looks
exactly the same. You can find more details in literature about
modelling, especially about the supercell method. (Take any book about
modelling of solid or liquids).

  Best regards


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

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