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The elements of coupled-cluster (CC) theory that distinguish it from other ways of treating the correlation problem are presented. These essential components required that new theory be developed to treat analytical gradients, excited states, higher order properties, and now multi-reference CC. Aided by these developments, CC theory provides the best answers for the largest number of problems in molecular structure and spectra. Many scientists who spent time at the Quantum Theory Project were instrumental in these developments.
Rodney J. Bartlett (Wed,) studied this question.