Spin-adapted multireference coupled-cluster approach: Linear approximation for two closed-shell-type reference configurations

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Abstract
An explicit form of the spin‐adapted multireference coupled‐cluster formalism in the linear approximation is developed for the special case of a two‐dimensional model space involving only closed‐shell‐type configurations. The formalism is applicable to a number of quasidegenerate systems with two valence orbitals of distinct spatial symmetry and should serve as a convenient testing ground for the suitability of the multireference coupled‐cluster theory. General problems of the multireference coupled‐cluster approach and its relationship with the corresponding configuration interaction formalism are discussed as well as the problems pertaining to a practical implementation of this formalism.