X-ray absorption spectrum of liquid water from molecular dynamics simulations: Asymmetric model

We address the question of the local structure in liquid water by computing the x-ray absorption (XA) spectrum explicitly averaging over configurations from classical and Car-Parrinello molecular dynamics (MD) simulations. We confirm the prediction by Wernet [Science 304, 995 (2004)] that the resulting spectra are not representative for liquid water due to a too large fraction of molecules with two donating hydrogen bonds. The H-bond criterion given by Wernet is, however, not sufficient to predict the XA spectrum for general structures from MD simulations. Only by selecting specific single-donor MD configurations is it possible to represent the liquid water spectrum; this results in an improved and more restrictive geometrical connection between local structure and spectral features in the simulation. The configurations, with one strong and one weak donating hydrogen bond, need to be even more asymmetric than previously assumed. This bonding situation is strongly underrepresented in the simulations.