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

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., alpha-->beta, excitations from a triplet (M-s=1) reference state for which both dynamical and nondynamical correlation effects are much smaller than for the corresponding singlet state. The simplest spin-flip model employs a 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. (C) 2002 American Institute of Physics.