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

[giacomo buccella]

Dear Charly,

my sistem is nearly 350 atoms. It is not that big, but enough to be sort of
unaffordable without using parallel machines, since I need to run tens of
MD runs considering different starting configurations. I surely can run one
test with a single thread, just to check how much the result is affected by
using long-range corrections.
Many thanks for your support
Best regards

Giacomo

Il giorno gio 17 nov 2022 alle ore 14:13 Carlos R. Lien-Medrano <
cmedrano at uni-bremen.de> ha scritto:

> 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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