I would like to simulate a system with only one planar electrode (graphene) in contact with electrolyte, and I want to use the ELECTRODE package. However, I have seen only simulations in the literature with two electrodes (maybe I have missed some). Thus, I was wondering if there is any issue if I use only one. I am able to run the simulation with only one electrode specifying the potential I want. My goal is to simulate the electrolyte at a specific electrode potential.
I have seen this previous post, but there is a recommendation to use two electrodes. However, it is not very clear why this is the case.
You can use “constrained charge” (from this paper) to simulate a single electrode at specified charge, and the charges on the electrode will redistribute themselves as needed to maintain equipotential. This is available through the fix electrode/conq command.
You are right that this technique has not been used often in simulations (although recently we applied it in this paper during parameter fitting). There are some important shortcomings:
You must use slab electrostatic boundary corrections and impose an artificial constraint, like a harmonic wall, on the “other end” of your solvent to keep it in the box. You must make sure that interface does not jeopardize your simulation results.
You must “match up” your imposed charge density with the electrode potential in some other way. This is a fundamental physical feature: an electrode’s charge can be unambiguously defined, but an electrode’s potential is only defined relative to some other reference.
In practice, you need to define integer charges on the electrode that are then balanced by adding counter ions to the electrolyte.
Thank you for your answer Shern! Your suggestions and the references to the papers are really valuable, so I know to use conq in this case.
Just one more thing out of curiosity. My initial guess was to use fix electrode/conp, since I would like to simulate at a specific electrode potential. However, for this specific system (one electrode) if I specify the potential in conp the electrode potential would not converge to the specified one, as far as I understand it. Is it correct?
There are two problems with using electrode/conp with a single electrode:
The electrode’s charge will continuously fluctuate. Since no other charges in the system are fluctuating, the system’s total charge will be generally non-zero and non-constant. This does not seem physical – I have not investigated deeply enough to know what exactly goes wrong, but I would doubt the result until proven correct.
More fundamentally the “potential” of a single object is arbitrary. It is only defined with reference to a zero potential. In theory, in an MD simulation, the electric potential in the “bulk” is accessible and can be used as potential zero. However, this is not comparable to either experiments (where the “bulk potential” cannot be measured – if you put in an electrode, you now have a new liquid-electrode junction potential to consider) or DFT (where length scales currently make it very difficult to simulate a meaningful bulk-interface-electrolyte system).