Two approaches for modeling of the transmembrane potential, as present in all eukaryotic cells, are examined in detail and compared with each other. One approach uses an externally applied electric field, whereas the other maintains an imbalance of ions on the two sides of a membrane. We demonstrate that both methods provide converged results concerning structural parameters of the membrane which are practically indistinguishable from each other, at least for monovalent ions. Effects of the electric field on the detailed molecular structure of the phospholipid bilayer are also presented and discussed. In addition, we achieve a considerable speed-up of the underlying molecular dynamics simulations by implementing the virtual interaction sites method for the Slipids force field.