In electrophysiology experiments, a voltage is applied to the cellular membrane by charging the membrane. One side of the membrane will then be enriched in cations and the other in anions.
Applying electrical stimuli to membrane systems in simulations that use full periodic boundary conditions on the other hand is far from straightforward. Because systems communicate in the direction normal to the membrane plane, separating ions on either sides of the membrane requires to isolate the system from its images.
Rather than doubling the size of the system, we have proposed to add a vacuum slab. As the membrane is a capacitor, it is possible to choose a charge imbalance to yield a transmembrane potential of desired magnitude.