A rule, complementary to the @AT06996@, of building up the @C01248@ of atoms and molecules: a maximum of two electrons can occupy an orbital and then only providing that the spins of the electrons are paired, i.e. opposed. The principle demands that the wavefunction for a many-electron system must be @A00478@ with respect to the permutation of the space-spin coordinates for every pair of electrons.
See also:
antisymmetry principle
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1956 [Terms] [Paper]