In a hydrogen bond the hydron is bonded to more than one other atom: X-H^...Y. A hdrogen bond is an attractive interaction between a proton donor X-H and a proton acceptor Y. Atoms X and Y are usually highly electronnegative N, O and F resulting in high dipole moments.  Hydrogen bonds are stronger than van der Waals forces, but weaker than covalent bonds.

Hydrogen bonding dominates a plethora of biological processes. On one hand it determines the structure of liquid water, the most important solute in nature, which plays a crucial role for reactions in the condensed phase. On the other hand hydrogen bonding determines the structures of proteins and DNA.

Large clusters may be suitable models to describe the transition from the gas phase to condensed phases. Our main goal is the determination of structural reorganization of the solvent shells depending on the number of solute molecules.

Further reading:

• P. Schuster, G. Zundel, C. Sandorfy,
  The Hydrogen bond Vol I-III ,
  North-Holland Publishing Company 1976
• C.L. Perrin and J.B. Nielson,
  "Strong Hydrogen Bonds"
  Ann. Rev. Phys. Chem. 48 (1998) 511.