A spin-flip variant of the second-order approximate coupled-cluster singles and doubles method

We report an implementation of a spin-flip variant of the second-order approximate coupled-cluster singles and doubles (CC2) method. The resolution-of-the-identity approximation or, alternatively, Cholesky decomposition are applied to the electron repulsion integrals. We illustrate the performance of the new method by constructing potential energy curves of H₂ and HF and by computing singlet-triplet splittings for various diradicals including some binuclear copper complexes that are of interest as molecular magnets. We find that spin-flip CC2 performs very similarly to the spin-flip variant of the algebraic diagrammatic construction scheme for the polarization propagator of second order (ADC(2)). Application to ozone shows that, depending on the reference state used, spin-flip CC2 erroneously predicts a barrierless symmetric dissociation of this molecule. A similar failure was observed in earlier work for spin-conserving CC2. In contrast, spin-flip ADC(2) and coupled-cluster singles and doubles yield qualitatively correct potential energy surfaces for ozone.