[DFTB-Plus-User] .cube file

Bálint Aradi balint.aradi at bccms.uni-bremen.de
Fri Dec 10 08:32:23 CET 2010


Dear Vladimir,

> Thank you, the previous suggestion has helped.  Now I can to obtain EP in my
> supercell containing the surface slab by myself just using net atomic
> charges. I also need the EP in presence of external field (I include
> external field in my calculations by placing point charges on the top layer
> of my slab). But if I set the Charge value in Hamiltonian is non 0 it also
> means that my surface is charged. What is the difference between this
> approaches (except the number of electrons = const)? I noticed that
> inclusion of point charges doesn't  change Mulliken charges of atoms
> significantly. For this reason obtained EP for charged surface is slightly
> differ than for neutral one (that looks strange). I afraid that something I
> do completely wrong either by including external field or by calculating EP.

 External charges are fixed point charges but not electrons! They do not
move, their charge does not change and they do not interact with each
other. The (external) potential generated by those charges is
incorporated in the Hamiltonian as an extra term. (Every electron in the
system will feel the additional potential they create.) If you plot the
EP potential in your system, you should add (additional to the Mullikan
charges on the atoms) the contribution from the external charges.

 "Internal" charges (set via the Charge keyword) are electrons in your
system. They move, they interact with each other, their distirbution is
determined by the SCC process. If you remove one of them, the
Hamiltonian will have the same form as before (no extra term appears),
only the concrete value SCC-term containing the interaction of the
electrons with each other will change, as you'll have a slightly other
charge distribution. The EP in the system will be determined purely by
the Mullikan charges.

 I hope, this clears the picture somewhat.

 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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