Perturbative corrections to the equation-of-motion spin-flip SCF model: Application to bond-breaking and equilibrium properties of diradicals

A. I. Krylov and C. D. Sherrill
J. Chem. Phys. 116, 3194 – 3203 (2002)

We present perturbative corrections to a recently introduced spin-flip self-consistent field (SF-SCF) wave function. The spin-flip model describes both closed and open shell singlet states within a single reference formalism as spin-flipping, e. g., α→β, excitations from a triplet (Ms = 1) reference state for which both dynamical and non-dynamical correlation effects are much smaller than for the corresponding singlet state. The simplest spin-flip model employs an SCF wave function for the reference state, and the resulting equations for target states are therefore identical to configuration interaction singles (in spin-orbital form). While being a qualitatively correct zero-order wave function, SF-SCF should be augmented by dynamical correlation corrections to achieve a quantitative accuracy. The results demonstrate that the second-order approximation to the more theoretically complete spin-flip coupled-cluster model (truncated at double substitutions) represents a systematic improvement over the SF-SCF model.

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