Hamaker constant

The Hamaker constant A can be defined for a Van der Waals (VdW) body–body interaction:

where ρ 1 and ρ 2 are the number densities of the two interacting kinds of particles, and C is the coefficient in the particle–particle pair interaction. It is named after H. C. Hamaker.

The Hamaker constant provides the means to determine the interaction parameter C from the Van der Waals pair potential, w ( r ) = − C / r 6 .

Hamaker's method and the associated Hamaker constant ignores the influence of an intervening medium between the two particles of interaction. In the 1950s Lifshitz developed a description of the VdW energy but with consideration of the dielectric properties of this intervening medium (often a continuous phase).

The Van der Waals forces are effective only up to several hundred angstroms. When the interactions are too far apart, the dispersion potential decays faster than 1 / r 6 ; this is called the retarded regime, and the result is a Casimir–Polder force.