In wavefunction theory, the correct spin symmetry of the many-particle wavefunction can be enforced through proper construction. For calculations performed using spin-unrestricted methods, spin contamination of the wavefunction can be measured by identifying contributions with higher spin multiplicity. In the case of Density Functional Theory (DFT), only the Kohn-Sham (KS) reference wavefunction is available, which pertains to a fictitious reference system of non-interacting particles. Consequently, in principle, it is not possible to accurately calculate two-particle quantities such as ⟨S 2 ⟩, even if the exact KS reference wavefunction was known.