A rule, complementary to the aufbau principle, of building up the electronic configuration 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 asymmetric 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 (https://doi.org/10.1351/pac199971101919)