Van der Waals forces are the only attractive forces present in nonpolar compounds.

They are weak interactions caused by momentary changes in electron density in a molecule. Thus they are the only force between non-polar molecules. Such as CH4 has no net dipole, at any one instant its electron density may not be completely symmetrical, resulting in a temporary dipole. This can induce a temporary dipole in another molecule. The weak interaction of these temporary dipoles constituents’ van der Waals forces.