Editing Intermolecular force (section)

===Keesom force (permanent dipole – permanent dipole) {{Anchor|Keesom force}}===
The first contribution to van der Waals forces is due to electrostatic interactions between rotating permanent dipoles, quadrupoles (all molecules with symmetry lower than cubic), and  multipoles.  It is termed the ''Keesom interaction'', named after [[Willem Hendrik Keesom]].<ref>{{cite journal| vauthors = Keesom WH |title=The second virial coefficient for rigid spherical molecules whose mutual attraction is equivalent to that of a quadruplet placed at its center |url=http://www.dwc.knaw.nl/DL/publications/PU00012540.pdf |journal= Proceedings of the Royal Netherlands Academy of Arts and Sciences |year=1915 |volume= 18 |pages= 636–646}}</ref> These forces originate from the attraction between permanent dipoles (dipolar molecules) and are temperature dependent.<ref name=j1/>

They consist of attractive interactions between dipoles that are [[canonical ensemble|ensemble]] averaged over different rotational orientations of the dipoles. It is assumed that the molecules are constantly rotating and never get locked into place. This is a good assumption, but at some point molecules do get locked into place. The energy of a Keesom interaction depends on the inverse sixth power of the distance, unlike the interaction energy of two spatially fixed dipoles, which depends on the inverse third power of the distance. The Keesom interaction can only occur among molecules that possess permanent dipole moments, i.e., two polar molecules. Also Keesom interactions are very weak van der Waals interactions and do not occur in aqueous solutions that contain electrolytes. The angle averaged interaction is given by the following equation:

:<math>\frac{-d_1^2 d_2^2}{24\pi^2 \varepsilon_0^2 \varepsilon_r^2 k_\text{B} T r^6} = V,</math>

where ''d'' = electric dipole moment, <math>\varepsilon_0</math> = permittivity of free space, <math>\varepsilon_r</math> = dielectric constant of surrounding material, ''T'' = temperature, <math>k_\text{B}</math> = Boltzmann constant, and ''r'' = distance between molecules.