[DFTB-Plus-User] How DFTB+ manages atomic charges?

[giacomo buccella]

Dear Bálint,
thanks for the kind reply. I've just tried to implement Long Range
corrections, but it seems that this option is not yet available when you
run with mpi threads:

**********************
...
Starting initialization...
--------------------------------------------------------------------------------
WARNING!
-> Dipole printed for a charged system : origin dependent quantity
WARNING!
-> Dipole printed for extended system : value printed is not well defined
ERROR!
-> Range separated calculations do not work with MPI yet

*********************
Do you have any other advice?

Many thanks
best regards

Giacomo

Il giorno gio 17 nov 2022 alle ore 07:54 Bálint Aradi <aradi at uni-bremen.de>
ha scritto:

> Dear Giacomo,
>
> this not DFTB related, but is common for all DFT approaches: DFT(B)
> methods tend to delocalize charges. You have one common Fermi level in
> the system, and if you have degenerated (or quasi-degenerated) states,
> such as likely the atomic level of the N atom and the VBM of your
> surface, charge will be distributed between them, independently, how far
> they are spatially from each other.
>
> Charge transfer reactions are usually calculated using range separated
> functionals (which you have in DFTB as well). Alternatively, one can
> also enforce a charge distribution via constraint DFT, but we don't have
> constrained DFTB in the official version yet.
>
> Best regards,
>
> Bálint
>
>
> On 16.11.22 11:16, giacomo buccella wrote:
> > Hi everyone, I'm asking for help about an issue to which I don't find
> > any answer in the manual.
> > I'm interested in simulating a chemical system with excess (or lack) of
> > charge on certain particular atoms.
> > The problem is that the after a single MD step (timestep=0.5 fs), the
> > charge is already delocalized among all atoms in the cell. This occurs
> > even if I'm simulating an ion approaching a distant surface.
> >
> > Example:
> > I want to simulate the ion N(+) approaching a surface, so I set the
> > initial charge of N atom = +1.000 :
> >
> > InitialCharges = {
> > AtomCharge = {
> > Atoms = N
> > ChargePerAtom = +1.0000
> > }
> > }
> >
> > therefore, I expect that at the beginning of the simulation the number
> > of electrons associated with N would be 5-1=4.
> > Well, after a single MD step, its charge is already about 4.9, even if
> > it is still 10 AA away from other atoms in the system with which it
> > could interact.
> >
> > My question is: how is this possible? is it something related to the way
> > DFTB+ manages the charge distribution? Do I need a time-dependent
> > calculation for this?
> > Any suggestion would be really appreciated
> > Many thanks
> >
> > Giacomo
> >
> >
> > _______________________________________________
> > DFTB-Plus-User mailing list
> > DFTB-Plus-User at mailman.zfn.uni-bremen.de
> >
> https://mailman.zfn.uni-bremen.de/cgi-bin/mailman/listinfo/dftb-plus-user
>
> --
> Dr. Bálint Aradi
> Bremen Center for Computational Materials Science, University of Bremen
> http://www.bccms.uni-bremen.de/cms/people/b-aradi/
>
>
> _______________________________________________
> DFTB-Plus-User mailing list
> DFTB-Plus-User at mailman.zfn.uni-bremen.de
> https://mailman.zfn.uni-bremen.de/cgi-bin/mailman/listinfo/dftb-plus-user
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.zfn.uni-bremen.de/pipermail/dftb-plus-user/attachments/20221117/05ac8f99/attachment.htm>