[DFTB-Plus-User] Charge density with DFTB+

[Daniele Selli]

Dear Balint,

> The big advantage of DFTB over DFT is speed, and parts of this speed
> advantage come from the simplified charge model. Using the explicit
> spatial-dependent density during the calculation is technically
> possible, but slow things down considerably, so it is questionable,
> whether it is worth the effort.

Ok, thank you for the clarification. Therefore, there is no actual
implementation of this kind of
electrostatic term calculation (spatial-dependent density - external point
charges)?

> Depending who you ask, the answer may be different. In my opinion,
> though, the Mulliken charges are the natural choice for being the basis
> of the electrostatics in DFTB.

> Is there any special reason, why the Mulliken-charges are not
> satisfactory for your purpose?

Usually, Mulliken charges are not that good and the use of other schemes to
assign proper atomic
charges is generally recommended (at least other DFT codes give the
possibility to evaluate the atomic
charges differently: ESP, NBO, CHelp...). I am just trying to understand if
there is the possibility of calculate
different kind of atomic charges with DFTB+.

Thank you again for your help!
Best Regards,

Daniele

Il giorno lun 3 giu 2019 alle ore 13:53 Bálint Aradi <aradi at uni-bremen.de>
ha scritto:

> Dear Daniele,
>
> > It is possible to have a real charge density (rho) of the system?
>
> Yes, for example waveplot is able to render the charge density of the
> system on a grid, if needed. You typically do it a posteriori, though.
>
> > I would like my system to interact with external point charges,
> > but, as far as I have understood, the coulomb interaction term is
> > between the Mulliken charges of the system and the external point
> > charges. Usually, in standard DFT, you would have an interaction between
> > the charge density (rho) of the system and the external point charges.
>
> The big advantage of DFTB over DFT is speed, and parts of this speed
> advantage come from the simplified charge model. Using the explicit
> spatial-dependent density during the calculation is technically
> possible, but slow things down considerably, so it is questionable,
> whether it is worth the effort.
>
> > If this is not the case, it would be possible to have other charges
> > (better than
> > the Mulliken one) from DFTB+?
>
> Depending who you ask, the answer may be different. In my opinion,
> though, the Mulliken charges are the natural choice for being the basis
> of the electrostatics in DFTB.
>
> Is there any special reason, why the Mulliken-charges are not
> satisfactory for your purpose?
>
>   Best regards,
>
>   Bálint
>
> --
> 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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