Hamaker constant

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

${\displaystyle A=\pi ^{2}\times C\times \rho _{1}\times \rho _{2}}$

where ${\displaystyle \rho _{1}}$ and ${\displaystyle \rho _{2}}$ are the number of atoms per unit volume in two interacting bodies and C is the coefficient in the particle-particle pair interaction.[1][2] 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, ${\displaystyle w(r)=-C/r^{6}}$.

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 ${\displaystyle 1/r^{6}}$; this is called the retarded regime and the result is a Casimir–Polder force.