7.11.7 Spin-Flip ADC Methods

The spin-flip (SF) method ⁶³⁶ Krylov A. I.
Chem. Phys. Lett.
(2001), 338, pp. 375. Link ^{, 637} Krylov A. I.
Chem. Phys. Lett.
(2002), 350, pp. 522. Link ^{, 638} Krylov A. I.
Acc. Chem. Res.
(2006), 39, pp. 83. Link ^{, 699} Lefrancois D., Wormit M., Dreuw A.
J. Chem. Phys.
(2015), 143, pp. 124107. Link 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. ⁶⁹⁹ Lefrancois D., Wormit M., Dreuw A.
J. Chem. Phys.
(2015), 143, pp. 124107. Link 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.