Atoms are surrounded by clouds of electrons. If they are forced together so closely that their electron clouds will overlap, they will strongly repel each other. This force is inversely proportional to r¹². Thus, it can be very significant at very short distances. The effectively creates a "wall" of repulsive force that prevents atoms (or molecules) from approaching closer than a minimum distance. This minimum distance for an atom is called the van der Waals radius, R. The closest distance two atoms can be from each other is R1 + R2, where R1 and R2 are the respective van der Waals radii of the atoms.