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I am trying to simulate the electron transfer from adsorbed H2O molecule to the electrode with AIMD. I guess I need to set up an external electric field, so the electron inside the H2O molecule would be driven to the electrode. But I haven't seen any paper to do this kind of simulation. My question is:

Is my thought appropriate to simulate the interfacial electron transfer in OER (oxygen evolution reaction)?

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  • $\begingroup$ Is it not enough to optimize the complex H2O+eletrode, calculate the charge distribution and then pick the charges from H2O after the optimization? If zero, there is no charge transfer, if positive, the water loss electron, if negative, the water gain electrons. $\endgroup$
    – Camps ♦
    Dec 20, 2022 at 12:41
  • $\begingroup$ If you are trying to apply an external electric field, it sounds like you’re trying to simulate charge transfer under an applied bias. Is this your intention? Thermodynamic calculations are fairly straight forward these days, where you try to get equilibrium charge distributions, but computing charge transfer barriers is more complicated. More advanced techniques may be required. You could also use implicit solvent methods with a grand canonical SCF algorithm to simulate constant potential results (if this is your end goal). $\endgroup$
    – Stephen
    Dec 22, 2022 at 19:31

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When a bias is applied to an electrode that drives OER, the bias is versus a counter electrode. The counter electrode is a macroscopic distance away, and I have yet to see an AIMD simulation that maintains distinct Fermi levels within one box. It is possible to apply an electric field in a simulation box, but I would be skeptical of the results. There are two other approaches you can take: driving the nuclear coordinate, and using the grand canonical ensemble.

Including an electric field

This is doable in common packages, such as VASP and CP2K. I doubt that confining such a field to the interface is implemented, and I am not sure what it would take to do this credibly.

Driving the nuclear coordinate

Within AIMD, the nuclear coordinate drives the electronic structure, under the assumption that the electronic degrees of freedom are in a ground state for the particular nuclear coordinates. So one can make the nuclear coordinates move and then the electron may at some point transfer. This is broadly under the umbrella of metadynamics. Common codes such as VASP and CP2K allow for metadynamics.

Here is an example: doi:10.1021/acs.jpcc.0c09108

Here is a CP2K tutorial: NHO3 on graphite

The downside is that to convince a reviewer that your sampling of pathways is comprehensive you will need multiple trajectories and most likely to simulate the reverse process as well.

Grand canonical

You can use the grand-canonical ensemble to include effects of solvents, capacitances, and biases. This method is ultimately closer to solving the structures statically and looking at the frontier orbitals. I am not an expert and cannot offer a more detailed explanation.

Here is an example: doi:10.1021/jacs.9b12474

Here is the method reference for VASP: doi:10.1021/acs.jctc.5b00170

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  • $\begingroup$ Could I simulate the applied voltage on the electrode by adding extra electrons in the system? suppose I only try to simulate a half-reaction. $\endgroup$
    – Jack
    Apr 10 at 20:18
  • $\begingroup$ @Jack you can add/subtract electrons to/from a system, such as by specifying NELECT in VASP to be explicitly different from the sum of the electron counts in the pseudopotential files. Extracting physical meaning from it is less trivial: the extra electrons may localize and definitely will interact with their own images in the periodic cells. This approach is used to model the energy levels of defect impurities in semiconductors - although rarely if ever benchmarked to experiment (see seminal works by Lany+Zunger 2009, Freyszoldt+Neugebauer around that time). $\endgroup$
    – Andrey Poletayev
    Apr 12 at 9:53
  • $\begingroup$ (cont'd) More recently, folks like David Scanlon automate it. What you get ultimately is the ability to solve for "at which chemical potential of electrons (≈ voltage vs some reference) does it become favorable for the structure to accept/donate electrons?" You can take this approach if you are trying to study, e.g., redox levels of localized surface states. This generally does not take into account the second party in the electron transfer that the original question you describe, and using it that way is conceptually most similar to the grand canonical approach in the original answer. $\endgroup$
    – Andrey Poletayev
    Apr 12 at 9:57

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