7.2.6 Spin-Adapted Spin-Flip CIS

Spin-Adapted Spin-Flip CIS (SA-SF-CIS) ¹³⁸⁰ Zhang X., Herbert J. M.
J. Chem. Phys.
(2015), 143, pp. 234107. Link is a spin-complete extension of the spin-flip single excitation configuration interaction (SF-CIS) method. ⁶⁵⁸ Krylov A. I.
Chem. Phys. Lett.
(2002), 350, pp. 522. Link Unlike SF-XCIS, SA-SF-CIS only includes the minimal set of electronic configurations needed to remove the spin contamination in the conventional SF-CIS method. The target SA-SF-CIS states have spin eigenvalues one less than the reference ROHF state, i.e., if singlet states ($S=0$) are targeted then the reference state should be a triplet ($S=1$), or if doublet states ($S=1/2$) are targeted then the reference state should be a quartet ($S=3/2$). The SA-SF-CIS approach uses a tensor equation-of-motion formalism, ¹³⁸⁰ Zhang X., Herbert J. M.
J. Chem. Phys.
(2015), 143, pp. 234107. Link such that the dimension of the CI vectors in SA-SF-CIS remains exactly the same as that in conventional SF-CIS. A DFT correction to SA-SF-CIS (i.e., SA-SF-TDDFT) can be added. ¹³⁸⁰ Zhang X., Herbert J. M.
J. Chem. Phys.
(2015), 143, pp. 234107. Link As with other SF-TDDFT methods, ¹³⁸¹ Zhang X., Herbert J. M.
J. Chem. Phys.
(2021), 155, pp. 124111. Link the BH&HLYP functional has become something of a de facto standard choice. ⁵⁰² Herbert J. M. et al.
Acc. Chem. Res.
(2016), 49, pp. 931. Link