Based on the superposition principle (the consequence of the linearity of Maxwell's equations), all possible exposure conditions from n electrodes can be computed using n-1 simulations 1. In these simulations, one electrode is assigned 1V Dirichlet boundary conditions, while the other ones are set to the ground. Linear combinations of these fields can then be found that inject/extract the desired currents at the desired active electrodes while ensuring that the other electrodes do not contribute current.
Electrode-skin interfaces were treated as ideal - i.e., perfect contact, no gel/paste, no electrode polarization, and no capacitive component.
1: Only n-1 simulations are needed, because adding a constant voltage offset does not modify the resulting fields, and because the amount of current entering the domain must exactly equal that exiting.