  X-ray absorption spectroscopy (XAS) is a powerful experimental technique to
probe the local order in materials with core electron excitations. Experimental
interpretation requires supporting theoretical calculations. For water, these
calculations are very demanding and, to date, could only be done with major
approximations that limited the accuracy of the calculated spectra. This
prompted an intense debate on whether a substantial revision of the standard
picture of tetrahedrally bonded water was necessary to improve the agreement of
theory and experiment. Here, we report a new first-principles calculation of
the XAS of water that avoids the approximations of prior work thanks to recent
advances in electron excitation theory. The calculated XAS spectra, and their
variation with changes of temperature and/or with isotope substitution, are in
excellent quantitative agreement with experiments. The approach requires
accurate quasi-particle wavefunctions beyond density functional theory
approximations, accounts for the dynamics of quasi-particles and includes
dynamic screening as well as renormalization effects due to the continuum of
valence-level excitations. The three features observed in the experimental
spectra are unambiguously attributed to excitonic effects. The pre-edge feature
is associated to a bound intramolecular exciton, the main-edge feature is
associated to an exciton localized within the coordination shell of the excited
molecule, while the post-edge one is delocalized over more distant neighbors,
as expected for a resonant state. The three features probe the local order at
short, intermediate, and longer range relative to the excited molecule. The
calculated spectra are fully consistent with a standard tetrahedral picture of
water.
