# Cooperation and Environment Characterize the Low-Lying Optical Spectrum of Liquid Water

## TL;DR

This study uses subsystem time-dependent density functional theory to analyze liquid water's optical spectrum, explaining features like broadening, shifts, and excitonic effects due to environmental interactions.

## Contribution

It provides a detailed theoretical explanation for the optical spectral features of liquid water, including environmental and many-body effects, which were previously elusive.

## Key findings

- Broader joint density of states causes red-shifted Urbach tail.
- First solvation shell induces blue shift of the absorption peak.
- Many-body excitonic effects influence low-frequency spectral weights.

## Abstract

The optical spectrum of liquid water is analyzed by subsystem time-dependent density functional theory. We provide simple explanations for several important (and so far elusive) features. Due to the disordered environment surrounding each water molecule, the joint density of states of the liquid is much broader than that of the vapor. This results in a red shifted Urbach tail. Confinement effects provided by the first solvation shell are responsible for the blue shift of the first absorption peak compared to the vapor. In addition, we also characterize many-body excitonic effects. These dramatically affect the spectral weights at low frequencies, contributing to the refractive index by a small but significant amount.