The spin-flip (SF) method
Chem. Phys. Lett.
(2001), 338, pp. 375. , 637 Chem. Phys. Lett.
(2002), 350, pp. 522. , 638 Acc. Chem. Res.
(2006), 39, pp. 83. , 699 J. Chem. Phys.
(2015), 143, pp. 124107. is used for molecular systems with few-reference wave functions like diradicals, bond-breaking, rotations around single bonds, and conical intersections. Starting from a triplet () ground state reference a spin-flip excitation operator is introduced, which flipped the spin of one electron while singlet and () triplet excited target states are yielded. The spin-flip method is implemented for the ADC(2) (strict and extended) and the ADC(3) methods. 699 J. Chem. Phys.
(2015), 143, pp. 124107. Note that high-spin () triplet states can be calculated with the SF-ADC method as well using a closed-shell singlet reference state. The number of spin-flip states that shall be calculated is controlled with the $rem variable SF_STATES.