van der Waals force
Van der Waals force is an attractive force existing, at very short distances, between atoms and molecules of all substances. It is named after Johannes van der Waals. The force arises as a result of electrons in neighboring atoms or molecules moving in sympathy with each other. This force is responsible for the term a/v2 in van der Waal's equation of state. In many substances it is small compared with other interatomic attractive and repulsive forces present.
Each atom has a characteristic van der Waals radius: the contact distance between any two atoms is the sum of their van der Waals radii. Two atoms will be attracted to each other by van der Waals forces until the distance between them equals the sum of their van der Waals radii.