• Strong spin correlation implies spin-symmetry breaking, though the converse is not true. In what follows, we will compute ⟨S 2 ⟩ UCCSD , as UCCSD does not contain free parameters and captures to a large extent what is frequently referred to as weak correlation along with some parts of strong correlation. Ref. 23 corroborates our hypothesis that the spin-symmetry breaking in UHF that remains after UCCSD can be attributed to strong spin correlations that are known to break low-order coupled cluster theory.