Multireference cluster expansion theory: MR–SAC theory

Multireference cluster expansion theory, called MR–SAC (multireference symmetry-adapted-cluster) theory, is presented. This theory is exact and unique, and yet does not include noncommutative algebra operator without imposing the completeness of the multireference space. The ansatz is simple enough for a general use in the study of potential energy surfaces of the ground and excited states of molecules. We have explained a recommended choice of the multireference operators on the basis of the analysis of an origin of the breakdown of the single reference theory. The method of solution of the MR–SAC theory is shown. The theory is given for the ground and excited states of closed-shell molecules, doublet and triplet states, and molecules with other symmetries. Test calculations are given for the ground and excited states of the CO molecule at the equilibrium and elongated distances.