Van der waals forces

Electronic charge around an atom fluctuates, which means that charge distribution isn't symmetrical. Atoms induce complementary asymmetry in neighbouring atoms. The atoms then attract each other. Attraction increases as the two atoms come closer together until they are separated by van der waals contact distance, which is where repulsive forces become dominant because the outer electron clouds of the two atoms overlap. The effect of van der waals forces is larger for larger molecules.