[CP2K-user] [CP2K:16664] Re: Modeling a charge-countercharge system

[adl233]

Hello CP2K Users,

I got the answer to this question courtesy of one of the authors of the 
above-mentioned article (Dr. Kristof Bal). I am sharing it here

The charge on the counterion can be controlled by not giving it any basis 
set. So, make it a "new" atom type that has no basis assigned to it. Here's 
an example: 

    &KIND D

      ELEMENT H

      BASIS_SET NONE

      POTENTIAL GTH-PBE-q1

    &END KIND

 This gives you a +1 ion, provided such a particle is placed in a vacuum. 
CP2K is an AO code, so it can only place electrons around cores that have 
basis functions around them. This is very different from PW codes like 
VASP. 

If you request a neutral system from CP2K, and include one such "D" atom in 
the system, you will have a -1 slab. After all, the one "hydrogen" electron 
cannot be placed around D, and must end up in the slab. 

You can get the opposite charge distribution by using the CORE_CORRECTION 
feature in CP2K to change the charge of the core of "D". But be aware that 
charge bookkeeping becomes tricky. For a +1 slab with -1 counterion, you 
must set CORE_CORRECTION to -2 (lower from +1 to -1) but CHARGE to +2. Why? 
Well, you want "one less" electron in the slab, but also not add the extra 
electron you would introduce by bringing in an extra "H". So, "not add" two 
electron => CHARGE +2.

Regards,

Aditya

On Thursday, March 3, 2022 at 9:24:56 AM UTC-5 adl233 wrote:

> Dear CP2K users,
>
> I apologize in advance if this is a trivial question. I am trying to model 
> a system where a slab surface is charged in a non-periodic cell and a 
> counterion is added to make the entire system charge neutral. I am trying 
> to follow the procedure from the following paper: 
> https://iopscience.iop.org/article/10.1088/1361-6595/aaa868/meta
>
> However, I do not understand the exact method by which this was 
> implemented. I read about core-correction option as well as CDFT and 
> confused about the right way to proceed. I am copying parts of the paper 
> relevant to this question below.
>
> "The practical realization of this approach in a standard DFT code (CP2K) 
> is as follows. The negative surface charges in this work require a positive 
> countercharge which, in the simplest case, can be a proton. It is, however, 
> not always straightforwardly possible to introduce gas phase ions of a 
> specified charge into the simulation box. This can be compensated by using 
> a DFT code that expands the Kohn–Sham orbitals in an atom-centered 
> (localized) basis: if no basis functions are added on the counterion, no 
> electronic density can spill over, its charge can be precisely controlled 
> and the desired surface charge can be enforced. The method is in principle 
> readily usable in any DFT code that uses localized basis sets (such as the 
> here used CP2K) but has, to the best of our knowledge, not yet been 
> described in the literature."
>
> and 
>
> "In the setup adopted in this work, a single additional electron is 
> considered and the countercharge (a proton) is placed at a Z position of 40 
> Å in a box of dimensions 16.1606 × 16.8106 × 100 Å3"
>
> Regards,
> Aditya
>

-- 
You received this message because you are subscribed to the Google Groups "cp2k" group.
To unsubscribe from this group and stop receiving emails from it, send an email to cp2k+unsubscribe at googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/cp2k/d568db0f-cba1-435e-a443-a0836a72ced1n%40googlegroups.com.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.cp2k.org/archives/cp2k-user/attachments/20220303/4416ca20/attachment.htm>