Singlet–triplet separation in BN and C2: Simple yet exceptional systems for advanced correlated methods

The singlet–triplet separation in the BN molecule, as well as in the isoelectronic C2, are examined via the recently developed reduced multi-reference coupled-cluster method with singles and doubles that is perturbatively corrected for triples, RMR CCSD(T), using the correlation consistent basis sets (cc-pVnZ, n = 2, 3, 4, and 5). Unlike the often used CCSD(T) method, which fails to predict the correct ground state for BN, the RMR CCSD(T) method yields a triplet ground state for BN and a singlet ground state for C2, as well as a realistic singlet–triplet separation energy.