EXTERNAL_POTENTIAL

[Бранислав Миловановић]

Dear cp2k community,

I'm confused about two EXTERNAL_POTENTIAL sections:

https://manual.cp2k.org/cp2k-9_1-branch/CP2K_INPUT/FORCE_EVAL/EXTERNAL_POTENTIAL.html

and

https://manual.cp2k.org/cp2k-9_1-branch/CP2K_INPUT/FORCE_EVAL/DFT/EXTERNAL_POTENTIAL.html

I notice differences between keywords and subsections and I want to simulate interaction of different solutes in the bulk water with the polarized graphene suface.

My section within the FORCE_EVAL:

  &EXTERNAL_POTENTIAL

    ATOMS_LIST 61..363                      # applied only on waters present in the system
    FUNCTION (A/B)*exp(-Z/Z0)         # potential that decays exponentially
    VALUES [eV] -3.0 [angstrom] 1.0 [angstrom] 6.0
    PARAMETERS A B Z0
  &END EXTERNAL_POTENTIAL

With this setup I noticed that the total energy is lower compared when no external potential is present and water molecules interact in the way I expected.

So, the questions are:
1) Is this the right way to simulate interaction with the electric field that originate from some surface, or should I employ EFIELD keywords or something similar?

2) Do I need to define EXTERNAL_POTENTIAL section within the DFT section as well, since I don't need some of the specific options there?

3) Could someone explain how PBC are entangled with the EXTERNAL_POTENTIAL?

Thanks,

Branislav

[Matt Watkins]

I'd need to check the exact details as I've forgotten:

The one inside the DFT section will operate on both electrons and atomic cores in a DFT calculation - so it pretty much corresponds to adding an electric field.

The one inside the FORCE_ENV I _think_ will only operate on cores and is mainly designed for use in the MM code or non-grid based methods. 

If you are doing a DFT calc you want the one inside the DFT section, almost certainly.

Matt

[Бранислав Миловановић]

Thanks a lot for clarifying!

If would be useful if someone added these into the reference manual description.