Equation-of-motion spin-flip coupled-cluster model with single and double substitutions: Theory and application to cyclobutadiene
While equation-of-motion coupled-cluster (EOM-CC) method is capable of describing certain multi-configurational wave functions within a single-reference framework (e. g. open-shell type excited states, doublet radicals, etc.), it may fail in cases of more extensive degeneracy, e. g. bond-breaking and poly-radicals. This work presents an extension of the EOM-CC approach to these chemically important situations. In our approach, target multi-configurational wave functions are described as spin-flipping excitations from the high-spin reference state. This enables a balanced treatment of nearly degenerate electronic configurations present in the target low-spin wave functions. The relations between the traditional spin-conserving EOM models and the EOM spin-flip method is discussed. The presentation of the formalism emphasizes the variational properties of the theory and shows that the killer condition is rigorously satisfied in single-reference EOM-CC theories. The capabilities and advantages of the new approach are demonstrated by its application to cyclobutadiene.