Hamaker summation

A pairwise-additive approximation integrating microscopic dispersion interactions over body volumes. For two nonretarded homogeneous half-spaces, it gives interaction energy per area $-A_H/(12\pi d^2)$.

The nonretarded attractive normal pressure obtained by differentiating the half-space interaction energy per area: $P=-A_H/(6\pi d^3)$. Total force additionally requires the interacting area.