Understanding X-ray absorption in liquid water: triple excitations in multilevel coupled cluster theory

TL;DR

This paper introduces a novel multilevel coupled cluster method to accurately simulate X-ray absorption spectra of liquid water, overcoming size limitations and providing detailed electronic structure insights.

Contribution

The authors develop and apply a new coupled cluster approach (MLCC3-in-HF) for large molecular systems, enabling precise simulation of liquid water's X-ray absorption spectrum.

Findings

Excellent agreement with experimental spectra achieved

Accurate first solvation shell electronic structure suffices for modeling

Reliable charge transfer analysis consistent with prior studies

Abstract

We present the first successful application of the coupled cluster approach to simulate the X-ray absorption (XA) spectrum of liquid water. The system size limitations of standard coupled cluster theory are overcome by employing a newly developed coupled cluster method for large molecular systems. This method combines coupled cluster singles, doubles, and perturbative triples in a multilevel framework (MLCC3-in-HF) and is able to describe the delicate nature of intermolecular interactions in liquid water. Using molecular geometries from state-of-the-art path-integral molecular dynamics, we obtain excellent agreement with experimental spectra. Additionally, we show that an accurate description of the electronic structure within the first solvation shell is sufficient to model the XA spectrum of liquid water. Furthermore, we present a rigorous charge transfer analysis with unprecedented reliability, achieved through MLCC3-in-HF. This analysis aligns with previous studies regarding the character of the prominent features of the spectrum.