Term: open-shell systems
https://doi.org/10.1351/goldbook.OT07087

Definition:
Atomic or molecular systems in which the electrons are not completely assigned to orbitals in pairs.

Source: PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1956 (https://doi.org/10.1351/pac199971101919)

Citation: 'open-shell systems' 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.OT07087

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