The changing hydrogen-bond network of water from the bulk to the surface of a cluster: a born-oppenheimer molecular dynamics study.

The effect of the environment on the properties of water in the bulk and at the surface of a cluster is studied by all-electron Born-Oppenheimer molecular dynamics. The vibrational spectrum of surface and bulk water is interpreted in terms of the molecular orientation and the local changes in the H-bond network of the cluster. Our results show that, in spite of the presence of a surface moiety of "acceptor-only" molecules, the H-bond network is significantly more labile at the surface than in the bulk part of cluster, and single donor-acceptor arrangements are largely dominant at the interface. Further, although surface water molecules depict in average a single H atom protruding into the vapor, molecules exhibit significant orientational freedom. These results explain the apparently opposite experimental observations from infrared sum frequency generation and X-ray spectroscopy of the liquid-vapor interface. The dipole moment, intramolecular geometry and surface relaxation are also analyzed at light of the different H-bond regions in the cluster.