I am not a physicist, and I'm not sure what keywords to use to search for this.

I'm trying to understand how I could in theory quantitatively know, what the voltage across a cell membrane in a Neuron would be, given complete information about the microstate of the system at the atomic/molecular level (positions of all the ions and so on). I am not even sure which information one needs to determine the voltage. E.g. do we need to know the nr of charged ions on both sides of the membrane? Do we need to divide by the volume of the neuron? Or by the surface area, given that the ions will concentrate near the boundary (i.e. near the membrane)? Does the distribution of ions within the neuron matter? Do we have to make approximations to make a prediction, or is there a simple exact relation?

Ideally I'm looking for a simple quantitative formula, but I'm not sure what the name is.