[Pw_forum] Image charge interaction
oliviero.andreussi at usi.ch
Wed Sep 14 14:06:13 CEST 2016
I think the discussion is quite full already, but maybe I can add a few details on the Environ module, which may be of some use in your application, similarly to what Giuseppe already said.
Environ allows to introduce a continuum dielectric medium in your simulation cell, this was shown to stabilise anions with respect to the calculation in vacuum (as a matter of fact small anions with plane waves are unbound and often crash with PW.x, while the same systems converge easily in dielectric media, even with small dielectric constants, with the extra electron happily sitting on the anion).
Moreover Environ allows to introduce the in-plane homogenous PBC correction for slabs (parabolic correction, I guess is something similar or the same thing of the dipolar correction that Thomas discussed, just with the extra capability of working also in the presence of the dielectric medium).
Eventually, Environ allows to introduce in the simulation cell one (or more) external charge distributions of arbitrary position and dimensionality (point-like, linear, planar) and of gaussian shape. Whether this is something useful in your case and the calculation would converge with it, is not immediately clear to me (especially if you want to place the extra charge inside the bulk of the system).
You can checkout the module and some more informations at www.quantum-environ.org <http://www.quantum-environ.org/>, unfortunately the documentation is still incomplete, but feel free to contact me if you need any help or additional information.
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> On 14 Sep 2016, at 10:46, 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...
> 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.
>> 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!
> 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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