text: van der Waals force is actually the total name of dipole-dipole force, dipole-induced dipole force and dispersion forces, in which dispersion forces are the most important part because they are always present.
  Assume that the pair potential between two atoms or small molecules is purely attractive and of the form w = −C/r, where C is a constant for interaction energy, decided by the molecule's property and n = 6 for van der Waals attraction. With another assumption of additivity, the net interaction energy between a molecule and planar surface made up of like molecules will be the sum of the interaction energy between the molecule and every molecule in the surface body. So the net interaction energy for a molecule at a distance D away from the surface will therefore be
  where 
  *  is the interaction energy between the molecule and the surface,
  *  is the number density of the surface,
  *  is the axis perpendicular to the surface and passesding across the molecule, with  at the point where the molecule is, and  at the surface,
  *  is the axis perpendicular to the  axis, with  at the intersection.
  Then the interaction energy of a large sphere of radius R and a flat surface can be calculated as
  where
  * W(D) is the interaction energy between the sphere and the surface,
  *  is the number density of the sphere.
  For convenience, Hamaker constant A is given as
  and the equation becomes
  With a similar method and according to Derjaguin approximation, the van der Waals interaction energy between particles with different shapes can be calculated, such as energy between
  * two spheres: 
  * sphere and surface: 
  * two surfaces:  per unit area.