Many molecules have permanent dipole moments, e. g. any heteronuclear di-
atomic like HF. But all molecules have transient dipole moments which arise because
the chance that all electrons, everywhere, will be exactly symmetrical about all axes in
space is... zero. This is the origin of the so-called London dispersion forces or Van der Waals
forces that cause molecules to always condense into a liquid phase when the tempera-
ture is lowered enough. The potential energy for a classical dipole at the origin, with the
charge distribution perturbed along the x-axis is