On the significance of quadruply excited clusters in coupled-cluster calculations for the low-lying states of BN and

The importance of higher than-pair clusters for the description of the lowest-lying singlet and triplet states of the BN and C 2 molecules via coupled-cluster (CC) approaches is examined using the reduced multireference (RMR) and completely renormalized (CR) CC methods, specifically the RMR CCSD(T) and CR-CC(2,n), n = 3 and 4 approaches. Special attention is paid to the BN molecule, to a comparison of the RMR CCSD(T) and CR-CC(2,3) methods, and to the importance of connected quadruply excited clusters for a proper description of the dependence of the singlet–triplet separation on the size of the basis set.