"The structure of water is the result of the balance between the thermal forces pulling water clusters apart and the chemical forces holding these clusters together. The dominant chemical force holding water molecules together is the hydrogen bond, … water is in a state of temperature-controlled equilibrium between the hydrogen bonded (HB) and the nonhydrogen bonded (nHB) forms. The nHB form is simply the free H2O molecule, and the various (HB) forms exist in a rapidly exchanging continuum of configurations of which the dominant form is the tetrahedral pentamer (H2O)5" . This pentamer with tetrahedral form is known as the Walrafen pentamer; the "phenomenon of dominant end members represented by the H2O and (H2O)5 forms connected by a continuum of rapidly exchanging intermediate forms (dimers, trimers, and tetramers) is essential for understanding the properties of water and sea water" .