# Molecular Polarizability of Water from the Local Dielectric Response Theory

## TL;DR

This paper introduces a fully ab initio local dielectric response theory to accurately compute water's molecular polarizability, revealing anisotropic effects and charge transfer contributions that enhance understanding of water's dielectric properties.

## Contribution

It develops a novel ab initio method based on local dielectric response theory to analyze electronic excitations in water beyond the dipole approximation.

## Key findings

- Hydrogen-bond network causes strong anisotropic effects on polarizability.
- Charge transfer increases isotropic polarizability by 5-6%.
- Out-of-plane component of polarizability is significantly enhanced.

## Abstract

We propose a fully ab initio theory to compute the electron density response under the perturbation in the local field. This method is based on our recently developed local dielectric response theory [Phys. Rev. B 92, 241107(R), 2015], which provides a rigorous theoretical framework to treat local electronic excitations in extended systems beyond the commonly employed dipole approximation. We have applied this method to study the electronic part of the molecular polarizability of water in ice Ih and liquid water. Our results reveal that the crystal field of the hydrogen-bond network has strong anisotropic effects, which significantly enhance the out-of-plane component and suppress the in-plane component perpendicular to the bisector direction. The contribution from the charge transfer is equally important, which increases the isotropic molecular polarizability by 5-6%. Our study provides new insights into the dielectric properties of water, which form the basis to understand the electronic excitations in water and to develop accurate polarizable force fields of water.