The interaction of two systems, such as atoms or molecules, by their dipole moments. The energy of dipole–dipole interaction depends on the relative orientation and the strength of the dipoles and how far apart they are. A water molecule has a permanent dipole moment, thus causing a dipole–dipole interaction if two water molecules are near each other. Although isolated atoms do not have permanent dipole moments, a dipole moment can be induced by the presence of another atom near it, thus leading to induced dipole–dipole interactions. Dipole–dipole interactions are responsible for van der Waals’ forces and surface tension in liquids.