VanderWaals forces

The Dutch researcher Dr. van der Waals described certain forces between molecules; attraction between molecules, even when these molecules as non polar.
The attraction between non polar molecules is sometimes called: vanderwaals forces.
But in other books, the vanderwaals forces can include polar as well as non polar attraction.
Never mind. If you just understand that there are intermolecular forces.
Particles attract each other because they have mass.
The masses of particles can, when approaching each other - cause a certain polarity in each other (called: induction).
That is responsible for temporarily dipole forces.
Not that we are talking about strong forces. And also they depend on the number of electrons in the particles and how they can be polarised.
It also depends on how big the particles are.
Generally spoken we may say: the vanderwaals forces are related with the molecular mass.

The molecules of the Halogenes have low melting points, but have different melting points.