Alternative perturbation theories for triple excitations in coupled-cluster theory

The dominant method of small molecule quantum chemistry over the last twenty years is CCSD(T). Despite this success, RHF-based CCSD(T) fails for systems away from equilibrium. Work over the last ten years has lead to modifications of CCSD(T) that improve the description of bond breaking. These new methods include CCSD(T), CCSD(2)T, CCSD(2) and CR-CC(2,3), which are new perturbative corrections to single-reference CCSD. We present a unified derivation of these methods and compare them at the level of formal theory and computational accuracy. None of the methods is clearly superior, although formal considerations favour CCSD(T) and computational accuracy for the systems considered favours CR-CC(2,3).