The spin-flip equation-of-motion coupled-cluster electronic structure method for a description of excited states, bond-breaking, diradicals, and triradicals

A. I. Krylov
Acc. Chem. Res. 39, 83 – 91 (2006)

The spin-flip approach to multi-reference situations (e. g., bond-breaking, diradicals, and triradicals) is described. Both closed and open shell low-spin states are described within a single reference formalism as spin-flipping, e. g., α→β, excitations from a high-spin reference state for which both dynamical and non-dynamical correlation effects are much smaller than for the corresponding low-spin state. Formally, the SF approach can be viewed as an Equation-of-Motion model where target states are sought in the basis of determinants conserving the total number of electrons but changing the number of α- and β-electrons.

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