There are several types of van der Waals attraction: dipole/dipole, dipole/induced-dipole and spontaneous-dipole/induced-dipole. The spontaneous-dipole/induced-dipole attractions are also known as London dispersion forces. LDF are surprisingly strong but are only short range forces $e . g .$ surface of neutral molecules or inert gases. All molecules have LDF and strength increases with the size/surface area of the molecule. In case of halogens, LDF increases as $F _{2}< Cl _{2}< Br _{2}< I _{2}$