  We introduce a novel approach to sample the canonical ensemble at constant
temperature and applied electric potential. Our approach can be
straightforwardly implemented into any density-functional theory code. Using
thermopotentiostat molecular dynamics simulations allows us to compute the
dielectric constant of nano-confined water without any assumptions for the
dielectric volume. Compared to the commonly used approach of calculating
dielectric properties from polarization fluctuations, our thermopotentiostat
technique reduces the required computational time by two orders of magnitude.
