Term: spin contamination
https://doi.org/10.1351/goldbook.ST07103

Definition:
In unrestricted Hartree–Fock method, the wavefunctions obtained are eigenfunctions of the Hamiltonian and the spin projection S 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 contamination.

Related Terms:
1) hartree (http://doi.org/10.1351/goldbook.H02747).
2) spin projection (http://doi.org/10.1351/goldbook.ST07106).
3) contamination (http://doi.org/10.1351/goldbook.C01295).

Source: PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1962 (https://doi.org/10.1351/pac199971101919)

Citation: 'spin contamination' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.ST07103

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