[Pw_forum] Image charge interaction

Ilya Ryabinkin igryabinkin at gmail.com
Wed Sep 14 14:02:06 CEST 2016


> I don't know of any way of introducing a image charge in the
> metallic slab by hand...

well, I would envision a "quick-and-dirty" implementation as follows:
determine the position of a "mirror plane", which has to be somewhere
closeby to the surface of a real slab, add classical forces to the
dynamics from the potential U(d) = -1/2d (in au) where d -- is a
normal distance "pseudoanion--mirror plane". Electronic structure
calculations are totally unaffected, merely the dynamics is. I can
hack verlet code and add these forces directly to the forces() array
in the propagation step.

The reason I'm looking for the problem---currently my Cl- pseudoanion
flies off the surface even if it has a tiny non-zero component of
velocity in z direction. The image charge would stop it (which is the
physics behind that I'm missing).

Of course, I'm totally for the "rigorous" solution of the problem,
which, I believe is the constrained DFT with U determined
variationally. But you and others already pointed out numerous related
issues this solution might bring in to the problem. I shall definitely
look at these options but later.

Thanks for the discussion anyway! If anyone has other ideas -- please,
join the discussion!



On Wed, Sep 14, 2016 at 4:46 AM, Thomas Brumme
<thomas.brumme at mpsd.mpg.de> wrote:
> Dear Ilya,
> Guiseppe is right and this is also the only way I think you can get
> the image charge. You have to use the neutral Cl (i.e. no core
> hole), put an extra electron, and force it to stay on Cl. This you
> can try with +U (or SIC? or constrained DFT?). Anyway, in this
> case you can end up with a dipole in the system and you need to
> switch on the dipole correction. And you might get problems with
> the homogeneous background if your vacuum is too large and
> the area of your slab too small - the electrons could spill into the
> minimum which is in the vacuum due to the background.
> I don't know of any way of introducing a image charge in the
> metallic slab by hand...
> Thomas
> On 09/13/2016 10:57 PM, Giuseppe Mattioli wrote:
>> Dear Ilya
>> I'm afraid there is not too much to add. You can test yourself the
>> effect of sa emiempirical application of U by monitorizing the
>> occupation of 3s and 3p Cl orbitals in some simple system like a Na
>> and a Cl atom far from bond distance or a Cl[H2O]6(-) cluster to see
>> when you get Cl(-). A polar solvent/medium stabilizes the excess
>> charge and screen the interaction with images (see the role of
>> dielectric constant in the old Makov-Payne paper) so that you should
>> be able to use lower U values. I've still not published the psII
>> calculations and I'm not sure that I will, because it turned out that
>> structural results are in a closer agreement with experiment by using
>> a GTO framework and COSMO as implicit dielectric environment.
>> HTH
>> Giuseppe
>> Quoting Ilya Ryabinkin <igryabinkin at gmail.com>:
>>> 2 Thomas:
>>>> Or you can also use the simple dipole correction which was actually
>>>> developed for charged (particles on) surfaces, see, e.g.,
>>> No, I think I wish the opposite: currently my pseudo-anion is neutral,
>>> but the *real* one should not be -- and, hence, image charge
>>> interaction is missing... I'd like to get it back.
>>> Of course, I it is possible and straightforward to keep an extra
>>> electron on chlorine and simulate the system with real -1 charge, that
>>> all what you've said are relevant and good.
>>> So, Giuseppe
>>>> I've used a semiempirical U value of 4 eV on the Cl 3p electrons to
>>>> keep together the electrons of Cl- in photosystem II.
>>> Could you elaborate more on that? Or may be you've published the
>>> results -- I could read and reference them.
>>> Thanks to all of you, colleagues!
>>> --
>>> I.
> --
> Dr. rer. nat. Thomas Brumme
> Max Planck Institute for the Structure and Dynamics of Matter
> Luruper Chaussee 149
> 22761 Hamburg
> Tel:  +49 (0)40 8998 6557
> email: Thomas.Brumme at mpsd.mpg.de
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                    Ilya Ryabinkin
                 Postdoctoral Scholar
          Physical and Environmental Sciences
           University of Toronto Scarborough

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