Anomalous perpendicular dielectric response of nanometer-thin water films*

The perpendicular dielectric response of a one nm-thin water film was experimentally found to be surprisingly low [Fumagalli et al, Science 360, 1339 (2018)], with a relative dielectric constant of 2.1. There had been predictions of such an effect from simulations [e.g. C Zhang, F Gygi, G Galli, J. Phys. Chem. Lett. 4, 2477 (2013)], and further simulations after the experiments corroborated them [e.g. G. Monet et al,Phys. Rev. Lett. 126, 216001 (2021)]. The problem is, however, far from understood, since the simulations describe a mostly (or even uniquely) orientational response of the water molecules, while the experimental response seems to be mostly electronic (1.8 is the bulk value of the high-frequency relative dielectric constant). Using molecular-dynamics simulations of such a water film over long trajectories obtained from a TIP4P/2005 model, the dielectric response for the model is compared with the one obtained using density-functional theory calculations sampling the same trajectory, both using explicit dipole versus applied field calculations. With the same technique, the high-frequency response is also calculated for the film. The results are quite revealing and offer a better understanding of this anomalous behaviour.