The hydrogen bond

The concept of the hydrogen bond is a century old but remains youthful because of its vital role in so many branches of science and because of continued advances in experiment, theory and simulation. We discuss the structural and energetic characteristics of normal hydrogen bonds X–H···Y as well as some exceptions to the normal, including proton-shared and ion-pair bonds. We consider the harmonic and anharmonic vibration of X–H in a variety of complexes, and demonstrate that there is no fundamental difference between blue-shifting and red-shifting bonds. We discuss water clusters and liquid water and indicate possible directions of future progress.