When two H2O or HF molecules approach each other, we know from the previous discussion that, there is a long-range attraction between the molecules which we can attribute to the London dispersion energy.
Unlike van der Waals molecules a new factor enters into play: polar molecules are stabilized in a molecular solid or liquid by the attractive interaction of oppositely charged ends of the molecule.
The hydrogen bond is a very important example of this polar attraction. In the vapour phase, the geometry of (H2O)2 can be determined by microwave spectroscopy and is shown on the next page.