Wigner-Witmer rules

A set of results found by applying group theory that states which molecular electronic states can exist, starting from the electronic states of the isolated atoms. The rules were stated for diatomic molecules by Hungarian-born US physicist Eugene Wigner (1902–95) and E. E. Witmer in 1928 and were subsequently extended to polyatomic molecules. The Wigner-Witmer rules are also known as correlation rules since they involve the correlation between atomic electronic states and molecular electronic states. They are useful in analysing the electronic spectra of molecules.