CCSD theory algorithm

As a result, one may justifiably extrapolate the (T) contribution from smaller basis sets than its CCSD counterpart in W1 theory, we extrapolate from the small and medium basis sets, and in W2 theory from the medium and large basis sets. This means that the most extensive basis sets in the calculations, namely large in W1 theory and extra large in W2 theory only require CCSD calculations, which are both much less expensive than CCSD(T) and much more amenable to direct algorithms such as those described in Refs. 40-41. 

Keywords Coupled-cluster theory Local correlation methods Cluster-inmolecule formalism Linear scaling algorithms Single-reference coupled-cluster methods CCSD approach CCSD(T) approach Completely renormalized coupled-cluster approaches CR-CC(2,3) approach Large molecular systems Bond breaking Normal alkanes Water clusters... 

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