Molecular dynamics simulations of surface-specific bonding of the hydrogen network of water: A solution to the low sum-frequency spectra

Vibrational spectroscopic features of a strong hydrogen (H) bonding network at the vapor/water interface are elucidated by using molecular dynamics (MD) simulation combined with quantum chemical calculations. The strong H-bonding coupling is properly described with quantum electronic effects, including charge transfer. By incorporating these effects in quantum mechanics/molecular mechanics calculations of water surface, we proved that the long-standing controversy on the low-frequency feature in the sum-frequency spectrum is naturally resolved. This feature originates from the significant anisotropic response of O-H vibrations in the H-bonding network at the surface. The anisotropy of the H-bonding network is also revealed in the MD result that the tangential H bonds are stronger than the normal ones at the interface, which is the origin of the high surface tension of water.