I am simulating a system, where water molecules are confined between two parallel plates. Plates, as well as the simulation box, are periodical in x and y direction, but fixed in z direction.
Plates are expected to maintain at the same electric potential, absolute value.
The third plate is added and try to make the system neutral, but it still fails. The failure is explainable, since the plates’ separation distance matters the charges on the plates.
May I know whether there is a way that I could constrain the absolute electric potential value of the first two plates, let the third one varies to make the system charge neutral?
Once an infinitely periodic system has net non-zero charge it is not obvious what the potential reference would be, since the potential does not vanish anywhere along the x-y plane and the potential along the z-axis would be different at + and - infinity and intimately dependent on the instantaneous electrolyte configuration.
Meanwhile, while you could construct a neutralising counterplane as you draw, the charge induced on either negative plate will now depend specifically on the exact distance between the neutralising plane and the other electrodes. So your results will not be generalisable to an experimental system (unless your experimental system itself somehow adopts this setup).
Now, the ELECTRODE package can do what you suggest. You simply need to set it up as:
fix conp plate1 electrode/conp -1 1.805 ... &
couple plate2 -1 ... &
couple plate3 0 symm on ... # or qtotal 0 in the latest versions
and if that gives you meaningful results then that’s fine – it’s your computer time, not mine.
But I would strongly recommend using more conventional approaches. Conventionally, most investigators set up a pair of electrodes with a ∆V of 1.0 V and extract the interfacial behaviour of only the negative electrode.
There have also been open-boundary papers which add (wlog) positive counterions to the electrolyte so that they can simulate just one negative interface at a prescribed surface charge density – this is doable using electrode/conq, and with a proper open boundary on the other end that does not disrupt the electrode-electrolyte interface, that would be a very interesting study which we need more of to make simulations more efficient.
As a reviewer, I would need a lot of convincing to accept that the three-plate setup you describe here is meaningful (the clincher of course being that it is faithful to experiment in some way that the other approaches are not). And I also know from past experience that most constant-potential reviewers are even less convinceable than I am.