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

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.