The CCSD curve is also repulsive, and CCSD(T) yields both less consistent and less accurate correlation energies recovering less than 70% of the total. Furthermore, in small basis sets FCI does not bind Be 2 . In sum, mean-field and even low-order coupled cluster theory are inadequate in describing the berylium dimer, and the most challenging regime is not at dissociation (though this is still non-trivial due to the near-degeneracy of the 2s and 2p atomic orbitals) but at the equilibrium bond length. As shown in Fig. , the pathological behavior at shorter bond lengths is correctly reflected by the 2 ∆ and ⟨S 2 ⟩ UCCSD ; however, the latter decays to zero in the limit of separated Be atoms, which in our view is unphysical: