Modeling a charge-countercharge system

[adl233]

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

[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