In addition, the 2 γ αβ and 2 δ αβ in HHeH follow a similar trend as in stretched H 2 , with quantitative differences introduced by the intermediate coupling of a third center. Likewise, the squared Frobenius norm of 2 ∆ approaches a value incrementally higher. While ⟨S 2 ⟩ UCCSD is now able to register a signal, it is slower to reflect the emergence of biradicaloid character in the region of 1.25-1.5Å, compared to 2 ∆ quantities. The beryllium dimer has been the focus of a barrage of experimental and theoretical studies, due in part to the incredibly small measured bond energy of 2.67 kcal/mol. An-other interesting feature is the quasi-degeneracy of the Be 2s and 2p atomic orbitals; 75 the mixing of these orbitals as the atom centers approach one another gives rise to a notoriously pathological electronic regime. Predictions on Be 2 were found to be sensitive to choices of active space -RHF and RCAS(4e,4o), yield qualitatively incorrect repulsive curves while RCAS(2e,2o) recovers the correct behavior at the cost of quantitatively incorrect energies.