Water is the most important liquid, and the nature of its structure remains a topic of keen debate and an active area of research [1–9]. Much of this debate centers around whether water has a mainly tetrahedral structure with a continuum of distorted hydrogen bonds, or if it contains a mixture of two distinct components. One major development in recent years is the application of inner-shell spectroscopic techniques, such as X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) at the oxygen K-edge to investigate the structure of water [2,10–12]. These methods can provide a direct structural probe of water, providing insight into the nature of its hydrogen bonding network. Theoretical studies play a critical role in these studies, since the analysis of the experimental data requires calculations to provide a link between the observed spectral features and the underlying structure. However, the simulation of the XAS or XES for liquid water presents a difficult challenge because it requires accurate molecular dynamics simulations to provide a correct description of the molecular structure coupled with accurate calculations of the spectral properties, i.e. excitation energies and line intensities. Furthermore, adequate sampling over molecular configurations also needs to be accounted for.
