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I am interested in performing activation energy barrier calculations using VASP (Vienna Ab initio Simulation Package) specifically for reactions involving electron exchange. However, I am uncertain about the appropriate methodology for modeling cations or anions within the VASP framework to accurately calculate activation energy barriers for these reactions.

Could someone provide guidance or resources on how to properly model cations or anions in VASP for activation energy barrier calculations, particularly in the context of electron exchange reactions? Any insights into the suitable input parameters, settings, or considerations would be greatly appreciated. Thank you.

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    $\begingroup$ Have you considered the NELECT tag? this tag would help you set the number of valence electrons, such that for cations, you may need to remove electrons, while for anions, you may need to add electrons. Let's say you have 10 valence electrons in total, set NELECT=9 and that's how you would simulate having a cation. $\endgroup$ Feb 23 at 1:48
  • $\begingroup$ Thanks for the info! $\endgroup$ Feb 23 at 14:51

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The short answer is, unfortunately, you cannot model electron transfer or exchange directly. While it is possible to specify a total charge on the system with NELECT, VASP will decide itself where the electrons in the system are going to go since it is a plane-wave code. That means you cannot really directly tell VASP which atom may be charged in which way. A second way to look at the problem is to realize that during an electron transfer reaction the system will transiently inhabit an excited state where nuclear coordinates are not relaxed to the electron that has just transferred.

If there is a correspondence between some local atomic environment feature, such as the positions of neighboring atoms in a solvation shell for classical Marcus theory, then it may be possible to induce a semi-particular charge state by creating that local environment. If the atomic positions are arranged in such a way that a ground state has well-defined charges, then the electronic solver is likely to find that ground state.

But finding a transition state or a reaction path is less well-defined, because of the excited state issue.

Related studies:

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