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.