Size-consistent wave functions for bond-breaking: The equation-of-motion spin-flip model
A new approach to the bond-breaking problem is proposed. Both closed and open shell singlet states are described 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. Formally, the new theory can be viewed as an Equation-of-Motion model where excited states are sought in the basis of determinants conserving the total number of electrons but changing the number of α and β electrons. The simplest member of the proposed hierarchy of approximations employs the Hartree-Fock self-consistent field model for the reference, and single spin-flipping excitations for the final states' description. A systematic improvement is achieved by using an optimized orbitals coupled-cluster doubles wave function for the reference, and single and double spin-flipping excitations for the final states. The results for single bond breaking (in HF) and diradicals (ethylene torsion) are presented. The excellent performance of both methods for these challenging problems demonstrates that employing the triplet reference, which can be treated accurately by single-reference models, results in a balanced description of the singlet ground state both at equilibrium (closed shell), and at the dissociation limit (open shell diradical). Initial benchmarks of the new models for excitation energies are also presented (Be atom).