Contour plot of the LUMO orbitals of a (H₂O)₁₃ cluster *

Water molecule has a very simple composition: 2 hydrogen atoms and 1 oxygen atom.
In MO (molecular orbital) theory, the 1s orbitals of the two H atoms along with the 2px
and 2py oribitals of O atom form 2a1 and 1b2 molecular orbitals. These two
molecular orbitals are the O-H bonding orbitals. The oxygen atom also have electrons
that are not bonded to H atoms and they form non-bonding orbitals 1a1 and 1b1.
A molecular orbital energy diagram of water is shown below.

Water molecule has a relatively large dipole moment of 1.855 Debye due to the large
difference in electronegativities between O and H atoms. However, surprisingly enough,
the average dipole moment of water molecules in clusters or in condensed phase
increased to 2.4-2.6 Debye. This is due the cooperative effect of water upon the
formation of clusters. Cooperative effect basically says that certain properties of a
single molecule will show dramatic change when they form clusters (or form
condensed phase). This is not just an addition of the magnitude of the corresponding
properties of molecules involved, rather, this is a consequence that molecules within
the cluster influence one another in a cooperative way and therefore, facilitate the
enormous change of the property.

* This plot was generated by Molden with the molecular orbitals calculated in Gaussian 98 program.