  The van der Waals interaction between two polarizable atoms is considered. In
three dimensions the standard form with an attractive $1/|R|^6$ potential is
obtained from second-order quantum perturbation theory. When the electron
motion is restricted to lower dimensions (but the $1/|R|$ Coulomb potential is
retained), new terms in the expansion appear and alter both the sign and the
$|R|$ dependence of the interaction.
