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7.11 Correlated Excited State Methods: The ADC(n) Family

7.11.7 Spin-Flip ADC Methods

(December 20, 2021)

The spin-flip (SF) method 600 Krylov A. I.
Chem. Phys. Lett.
(2001), 338, pp. 375.
Link
, 601 Krylov A. I.
Chem. Phys. Lett.
(2002), 350, pp. 522.
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, 602 Krylov A. I.
Acc. Chem. Res.
(2006), 39, pp. 83.
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, 662 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 (ms=1) ground state reference a spin-flip excitation operator {C^J}={caβciα; caβcbσciαcjσ, a <  b, i <  j} is introduced, which flipped the spin of one electron while singlet and (ms=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. 662 Lefrancois D., Wormit M., Dreuw A.
J. Chem. Phys.
(2015), 143, pp. 124107.
Link
Note that high-spin (ms=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.