The Hamaker constant A can be defined for a van der Waals (vdW) body-body interaction:
where and are the number densities of the two interacting kinds of particles, and C is the London coefficient in the particle-particle pair interaction. It is named after H. C. Hamaker. The magnitude of this constant reflects the strength of the vdW-force between two particles, or between a particle and a substrate.
The Hamaker constant provides the means to determine the interaction parameter C from the vdW-pair potential, .
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 ; this is called the retarded regime, and the result is a Casimir-Polder force.