Theoretical approximations to X-ray absorption spectroscopy of liquid water and ice

We review methods to compute X-ray absorption spectra (XAS) with special focus on the transition-potential approach of Triguero et al. [Phys. Rev. B 58 (1998) 8097] and its application to calculations on water in condensed phase. We discuss the absolute energy scale, functional dependence, broadening versus sampling of intra- and intermolecular vibrational modes, treatment of the continuum, cluster size convergence as well as compare with periodic calculations and with experiment; periodic and cluster model calculations are found to agree very closely in the relevant near-edge region although neither reproduces the pre-edge and main-edge features in the experimental spectra of thin ice films. The real-space grid representation of the wave function in the periodic calculations allows a more extended energy range to be described and we find satisfactory agreement with experiment for higher energy continuum resonances. Two proposed alternative approaches using either the potential from a full core-hole (FCH) or the full core-hole with an excited electron in the lowest state (XCH) are shown to lead to spectra that deviate significantly from experiment.