7.9.5 Spin-Flip ADC Methods

The spin-flip (SF) method^{501, 502, 503, 556} 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 ($m_s=1$) ground state reference a spin-flip excitation operator $\{{\widehat{C}}_J\}=\{c_{a\beta }^{†}{c}_{i\alpha };$ $c_{a\beta }^{†}{c}_{b\sigma }^{†}{c}_{i\alpha }{c}_{j\sigma },$ $\text{a < b, i < j}\}$ is introduced, which flipped the spin of one electron while singlet and ($m_s=0$) triplet excited target states are yielded. The spin-flip method is implemented for the ADC(2) (strict and extended) and the ADC(3) methods.⁵⁵⁶ Note that high-spin ($m_s=1$) 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.