spin contamination

https://doi.org/10.1351/goldbook.ST07103
In unrestricted @[email protected]–Fock method, the wavefunctions obtained are eigenfunctions of the Hamiltonian and the @[email protected] \(S_{\text{z}}\) operators, but not eigenfunctions of the \(S^{2}\) operator. As a result, the wavefunctions of the doublet systems are spin-contaminated to some extent by admixtures of quartet, sextet, and higher spin states. The eigenvalues of the \(S^{2}\) operator are given as a measure of the spin @[email protected]
Source:
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1962 [Terms] [Paper]