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

[Carlos R. Lien-Medrano]

Dear Giacomo,

I don't know the dimensions of your system but is it strictly needed that
you run
the calculation in parallel? Otherwise, you could give a chance performing
the
calculation just in one node using long-range corrections.

Best regards,
Charly

El jue, 17 nov 2022 a las 7:02, giacomo buccella (<gek.buccella at gmail.com>)
escribió:

> 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/
>>
>>
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