  We study the ground state energy for a system of two hydrogen atoms coupled
to the quantized Maxwell field in the limit $\alpha \to 0$ together with the
relative distance between the atoms increasing as $\alpha^{-\gamma} R$, $\gamma
> 0$. In particular we determine explicitly the crossover function from the
$R^{-6}$ van der Waals potential to the $R^{-7}$ retarded van der Waals
potential, which takes place at scale $\alpha^{-2} R$.
