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

[Bálint Aradi]

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
> 
> 
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-- 
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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