New Insights into the Structure of the Vapor/Water Interface from Large-Scale First-Principles Simulations.

We present extensive ab initio simulations of the molecular arrangements at the vapor/water interface, which provide valuable insights into the interface structure. In particular, the simulations address the controversy of whether there is a significant amount of nondonor configurations at this prototypical interface, using a novel Car-Parrinello-like ab initio molecular dynamics approach. The interface is modeled by a system of 384 water molecules for 125 ps in a two-dimensional periodic slab, the most extensive ab initio molecular dynamics simulation to date. In contrast to previous theoretical simulations and X-ray absorption spectroscopy, but consistent with sum-frequency generation experiments, we observe no evidence for a significant occurrence of acceptor-only species at the vapor/water interface. Besides a distinct surface relaxation effect, we find that only the topmost layers of the interface obey structural order.