# Could I simulate the applied voltage on the electrode by adding extra electrons in the system?

I try to use molecular dynamics to simulate the half-reaction of the H2O reduction on the Pt cluster. Is it possible to add extra electrons in the system to simulate the reduction process?

The Pt cluster contains about 600 atoms, I found that if I put one electron in the system, the fermi energy could rise by about 1eV, but if I put five electrons into the cluster, although the water molecule would dissociate, but the fermi energy would rise by 8eV, which too much higher than experimental values.

My question is: Is it feasible to simulate the reduction process by adding extra electrons?

• Classic molecular dynamics did not consider the electrons, so, it can not be used to simulate any process where the electrons are involved. Instead, you can use Ab Initio Molecular Dynamics (AIMD). If you have an applied voltage, the electrons inside your system remains constant BUT now they are out of equilibrium and an electronic current appears (as the applied voltage creates an electric field that act on the electrons accelerating them).
– Camps
Apr 10, 2023 at 20:39
• @Camps， I am using Ab initio MD, but my concern is that only putting one extra electron would cause the fermi level to rise too much. so I am sure how to replicate the experimental applied voltage.
– Jack
Apr 11, 2023 at 4:19

You have found for yourself that it is not difficult to reduce water if you throw a suitably overwhelming number of electrons at it, but you're also skeptical that this reflects realistic experimental conditions. This means your physical intuition is working well!

The technique you are looking for is called constant potential DFT calculations, where -- precisely because you cannot trust what number of electrons you should throw into your system -- you control the work function of the electrode of interest, in effect choosing the electrode potential, and let the DFT package of choice use that to determine how many electrons are in the system and thus how happy water is to be reduced.

(Note that DFT people like to call it "grand-canonical ensemble" DFT, or GCE-DFT for short, because it studies the grand canonical ensemble for electrons. I dislike the term personally because some DFT practitioners refer to implicit solvent DFT as using a grand canonical ensemble for solvent molecules, and that's one too many grand canonical things for my liking. But you should probably Google GCE-DFT and see what you find.)

I've been reading these papers to get a handle on the technique, so perhaps they will be helpful to you too:

https://pubs.acs.org/doi/10.1021/acs.jpcc.8b10046

https://pubs.acs.org/doi/10.1021/acs.jctc.9b00717

https://pubs.acs.org/doi/10.1021/jacs.8b03002

https://pubs.acs.org/doi/10.1021/acs.chemrev.1c00981

https://aip.scitation.org/doi/full/10.1063/5.0138197 (fresh off the press!!)

and this link on one implementation (solvated jellium in GPAW):

https://wiki.fysik.dtu.dk/gpaw/documentation/sjm/sjm.html

All the best!