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# 7.11.7 Spin-Flip ADC Methods

(July 14, 2022)

The spin-flip (SF) method 636 Krylov A. I.
Chem. Phys. Lett.
(2001), 338, pp. 375.
, 637 Krylov A. I.
Chem. Phys. Lett.
(2002), 350, pp. 522.
, 638 Krylov A. I.
Acc. Chem. Res.
(2006), 39, pp. 83.
, 699 Lefrancois D., Wormit M., Dreuw A.
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 ($m_{s}=1$) ground state reference a spin-flip excitation operator $\{\hat{C}_{J}\}=\{c_{a\beta}^{\dagger}c_{i\alpha};$ $\hskip 2.845276ptc_{a\beta}^{\dagger}c_{b\sigma}^{\dagger}c_{i\alpha}c_{j% \sigma},$ $\hskip 7.113189pt\text{a \textless~{} b, i \textless~{} j}\}$ 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.
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.