  The retarded van-der-Waals potential, as first obtained by Casimir and
Polder, is usually computed on the basis of nonrelativistic QED. The
hamiltonian describes two infinitely heavy nuclei, charge $e$, separated by a
distance $R$ and two spinless electrons, charge $-e$, nonrelativistically
coupled to the quantized radiation field. Casimir and Polder use the dipole
approximation and small coupling to the Maxwell field. We employ here the full
hamiltonian and determine the asymptotic strength of the leading $-R^{-7}$
potential, which is valid for all $e$. Our computation is based on a path
integral representation and expands in $1/R$, rather than in $e$.
