[QE-users] [EXT] Charged Systems and PBC Corrections

[Andreussi, Oliviero]

Hi,

I am not 100% sure about the question, but my interpretation is that you want to compute adsorption energies of charged systems in a continuum environment. If you use different environ types (water and water-cation/anion) for the different systems, I would not feel comfortable with comparing the energies. You probably want to use the same model of continuum water in all the different calculations.

The question is, if some systems are neutral and some are charged, which environment is the right one? If you only care about cations, I would use the parametrization of the neutral systems (water), which gives pretty good results for cations as well.

If you are dealing with anions, the problem is more tricky, as the parametrization is very different and the neutral one significantly underestimate solvation of anions. One explanation for the need of a different parametrization is that water-anion interactions are stronger and possibly non-electrostatic (vdW) effects are more relevant. In this situation I would suggest to add some explicit water molecules on the negative residues of the anions and perform the continuum calculations with the neutral parametrization.

I hope this helps, if you have more doubts or questions, don’t hesitate to ask.

Best,

Oliviero

Oliviero Andreussi
--
Assistant Professor
Department of Physics
University of North Texas
Email: oliviero.andreussi at unt.edu<mailto:oliviero.andreussi at unt.edu>
Phone: +1-(940)-369-5316
Skype: olivieroandreussi
Web: https://www.materialab.org

On Oct 2, 2020, at 9:16 AM, Robert Stanton <stantor at clarkson.edu<mailto:stantor at clarkson.edu>> wrote:

Dear QE Users,

     Previously I got some great advice about dealing with charged systems by use of hybrid XC, use of implicit solvents through the Environ plugin, or alternatively use of the MT correction. I just wanted to double check something regarding this as reproducing literature values is consistent within certain systems for me, but not others.
      When using the Martyna-Tuckerman correction, other than making sure to use cell sizes of double the largest system in all directions, are there any concerns when comparing energies between states of different formal charge (other than potentially unbound electrons in the charged case)? For example when calculating charged molecule adsorption energies on neutral bodies of the form E_ads = E_neutral+charged - E_neutral - E_charged?
      Following the same sort of calculation above, would the correct approach be in using the energies from the two different flags 'water' and 'water-anion/cation' when actually using the parabolic correction+implicit solvent method implemented in Enviroin? Thank you all in advance!

Regards,
Robert Stanton
Graduate Student
Clarkson University


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