  In recent years, constant applied potential molecular dynamics has allowed to
study the structure and dynamics of the electrochemical double-layer of a large
variety of nanoscale capacitors. Nevertheless it remained impossible to
simulate polarized electrodes at fixed total charge. Here we show that
combining a constant potential electrode with a finite electric displacement
fills this gap by allowing to simulate open circuit conditions. The method can
be extended by applying an electric displacement ramp to perform computational
amperometry experiments at different current intensities. As in experiments,
the full capacitance of the system is obtained at low intensity, but this
quantity decreases when the applied ramp becomes too fast with respect to the
microscopic dynamics of the liquid.
