ABSTRACT
The single-reference formulation of the coupled cluster (CC) method has been appreciated as an effective and useful approach for computing electron correlation effects when a single determinant provides a good approximation to the exact function in the zero-order model. If this is not the situation, then a generalization toward a multireference (MR) CC formulation is needed. MRCC methodologies can be broadly classified into two categories depending on the choice of ansatz for the ground and excited states. In the single-reference based approach, the ground state is represented by an exponential CC ansatz, whereas the excited states are generated by the action of an excitation operator on the correlated ground state. A plethora of multireference methods within the single-reference coupled cluster framework have been proposed over the years to deal with situations where a single-reference function is inadequate to describe the state of interest. The methods which have been put
forward as remedies to this problem include symmetry adapted cluster (SAC) configuration interaction (CI) [135, 136], equation of motion coupled cluster (EOMCC) [137, 138, 139, 140, 141], coupled cluster based linear response theory (CCLRT) [142, 143], spin-flip EOMCC [144, 145], similarity-transformed Hamiltonian [146], method-of-moment (MM) coupled cluster [147], and stateselective multireference coupled cluster methods [148].
