Standard model for liquid water withstands x-ray probe

We present a series of ab-initio calculations of spectroscopic properties of liquid water at ambient conditions. Our results show that all available theoretical and experimental evidence is consistent with the standard model of the liquid as comprising molecules with approximately four hydrogen bonds. In particular, this model cannot be discounted on the basis of comparisons between measured and computed x-ray absorption spectra (XAS), as recently suggested. Our simulations of ice XAS including the lowest lying excitonic state are in excellent agreement with experiment and those of the TIP4P model of water are in reasonable agreement with recent measurements. Hence we propose that the standard, quasi-tetrahedral model of water, although approximate, represents a reasonably accurate description of the local structure of the liquid.