text: Molecules interact at close range through intermolecular forces (the "van der Waals 
  forces"), which cause minute shifts of the electron density distributions (relative 
  the distributions of electrons when the molecules are not interacting). 
  Intermolecular forces are repulsive at near range, where electron exchange 
  forces dominate the interaction, and attractive at somewhat greater separations, 
  where the dispersion forces are active. (If separations are further increased, all 
  intermolecular forces fall off rapidly and may be totally neglected.) 
  Repulsion and attraction are due, respectively, to the small defects or 
  excesses of electron densities of molecular complexes in the space 
  between the interacting molecules, which often result in interaction-induced 
  electric dipole moments that contribute some to interaction-induced emission and 
  absorption intensities. The resulting dipoles are referred to as exchange 
  force-induced dipole and dispersion force-induced dipoles, respectively.
  Other dipole induction mechanisms also exist in molecular (as opposed to 
  monatomic) gases and in mixtures of gases, when molecular gases are present. 
  Molecules have centers of positive charge (the nuclei), which are surrounded by
  a cloud of electrons. Molecules thus may be thought of being surrounded by various 
  electric multipolar fields which will polarize any collisional partner 
  momentarily in a fly-by encounter, generating the so-called multipole-induced 
  dipoles. In diatomic molecules such as H and N, the lowest-order 
  multipole moment is the quadrupole, followed by a hexadecapole, etc., hence the 
  quadrupole-induced, hexadecapole-induced,... dipoles. Especially the former is often
  the strongest, most significant of the induced dipoles contributing to CIA and CIE. 
  Other induced dipole mechanisms exist. In collisional systems involving 
  molecules of three or more atoms (CO, CH...), collisional frame 
  distortion may be an important induction mechanism. Collision-induced 
  emission and absorption by simultaneous collisions of three or more particles 
  generally do involve pairwise-additive dipole components, as well as important 
  irreducible dipole contributions and their spectra.