 If no other force is present, the distance between atoms at which the force becomes repulsive rather than attractive as the atoms approach one another is called the van der Waals contact distance; this phenomenon results from the mutual repulsion between the atoms' electron clouds.The van der Waals force has the same origin as the Casimir effect, arising from quantum interactions with the zero-point field.The term "van der Waals force" is sometimes used loosely for all intermolecular forces.