The square dissociation of H 4 is a notoriously difficult model problem for strongly correlated electrons. The exact ground state switches from triplet to singlet spin multiplicity at around 1.15Å, and in what follows we focus on the region of the curve with a singlet ground state. Figure highlights the large errors encountered for RCCSD and RCCSD(T) models, which can be understood by the inability to capture the θ 0,0 4,2 configuration (Eq. 13).