Ab-initio simulations of liquid water: dielectric and hydrophobic effects*

Hydrogen bonds are at the origin of many special properties of water. In this talk I will show how extensive \textit{ab-initio} molecular dynamics simulations and simple theoretical models give insight on the structure and dynamics of the H-bond network. I will discuss in particular: (a) a study of the dielectric properties of water, based on simulations under a finite applied electric field, and (b) a study of the molecular origin of the hydrophobic effect, based on simulations of a solvated methane pair.