Accurate calculation of the dielectric constant of water from simulations of a microscopic droplet in vacuum

We present a successful calculatioon of the static dielectric constant of water based on simulations of microscopic droplets in vacuum. A 1.5 ns simulation of a 24 Å radius sphere of TIP3P water gives ϵ = 82 ± 5 at 292 K; a 2 ns simulation of a 14 Å sphere gives ϵ = 82 ± 5 at 294 K. Inclusion of all electrostatic interactions in the droplet was critical to success. Polar fluctuations in the inner half of the droplet are bulk-like, due to efficient screening by the outer half, despite the surrounding vacuum. This suggests that droplet simulations of more complex systems, such as proteins in water droplets, can successfully account for solvent screening of charges near the center of the simulation sphere.