[CP2K:2703] Reference point for molecular dipoles

[Juerg Hutter]

Hi

the total dipole is given as the sum of electronic and nuclear
contributions. Both contributions depend on the center of
origin and for neutral molecules this dependence cancels.
For ions this is not the case and you geta dependence on the
reference point. COAC (center of atomic charges) was chosen
as the reference point as it fully cancels the contribution
from the ions and leaves only the electronic contribution.
In other words, it minimizes the size of the electronic part
of the dipole.
Your suggestion would achieve the same but with the nuclear
contribution, the electronic contribution would be zero and
the nuclear minimal.
I prefere the COAC reference point as it only depends on
the molecular coordinates (and ionic charges). If you calculate
the same molecule with a different electronic structure method
you will have the same reference point and you can compare
the results.

regards

Juerg

----------------------------------------------------------
Juerg Hutter                   Phone : ++41 44 635 4491
Physical Chemistry Institute   FAX   : ++41 44 635 6838
University of Zurich           E-mail: hut... at pci.uzh.ch
Winterthurerstrasse 190
CH-8057 Zurich, Switzerland
----------------------------------------------------------


On Mon, 10 May 2010, Ondrej Marsalek wrote:

> Dear all,
>
> I am looking at the key:
>
> CP2K_INPUT / FORCE_EVAL / DFT / PRINT / LOCALIZATION /
> MOLECULAR_DIPOLES / REFERENCE
>
> and one of the possible values is "COAC", with the description "Use
> Center of Atomic Charges". I would like to ask how are these atomic
> charges calculated. It would be nice to have the option to simply use
> the center of charge assigned to that molecule based on the Wannier
> centers, but that seems to be missing, as far as I can tell. I want to
> get the molecular dipole of an ion, so the default value "ZERO" is no
> good and center of charge seems a better choice than center of mass.
>
> Thanks,
> Ondrej
>
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