**Exchange interaction** Exchange interaction (Pauli repulsion) is a purely quantum mechanical effect without any analog in classical mechanics.

It is the result of the fact that the wave function of indistinguishable particles is subject to exchange symmetry, that is, the wave function describing two particles that cannot be distinguished must be either unchanged (symmetric) or inverted in sign (antisymmetric) if the labels of the two particles are changed.

An interaction represented by a potential involving exchange of space or spin coordinates, or both, of the particles involved

__Exchange repulsion__ The correlation to the Coulomb repulsion between two electrons in orbitals Ci and Cj for the case when the electrons possess parallel spins. It is to be substracted from the Coulomb repulsion to give the total energy of the electron-electron interaction. In the Hartree-Fock theory the magnitude of the exchange repulsion is given by the exchange integral

Kij = <ij|ji>

For the case of electrons with opposite spins Kij vanishes.

__See also:__ Exchange symmetry

http://www.oraxcel.com/cgi-bin/yabb2/YaBB.pl?num=1342073993/0#0 __Source:__ http://en.wikipedia.org/wiki/Exchange_interaction http://www.answers.com/topic/exchange-interaction 1999 IUPAC, Pure Appl. Chem. 71, 1938