The in-plane dielectric constant of nanoconfined water has attracted growing interest in recent years. Nevertheless, assigning a bulklike dielectric constant to a molecularly thin film is convention dependent because it requires choosing an effective water width. We propose the in-plane 2D polarizability, ${\alpha }_{\parallel }$, as a macroscopic characterization of the in-plane dielectric response that avoids this choice. Using classical molecular dynamics simulations, we compute ${\alpha }_{\parallel }$ via two independent and consistent methods: based on fluctuation-dissipation theory, and from the induced dipole moment when water is placed in a capacitor. Our results provide the framework to quantify the in-plane dielectric response of polar liquids across simulations and experiments.

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• Classical statistical mechanics
• Dielectric properties
• Fluctuation-dissipation theorem
• Interface & surface thermodynamics
• Permittivity
• 2-dimensional systems
• Polarizability
• Polarization
• Molecular dynamics