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.
