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

[giacomo buccella]

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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