Spin-Adapted Spin-Flip CIS (SA-SF-CIS)1102 is a spin-complete extension of the spin-flip single excitation configuration interaction (SF-CIS) method.510 Unlike SF-XCIS, SA-SF-CIS only includes the minimal set of necessary electronic configurations 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. Based on a tensor equation-of-motion formalism,1102 the dimension of the CI vectors in SA-SF-CIS remains exactly the same as that in conventional SF-CIS. Currently, the DFT correction to SA-SF-CIS is added in an ad hoc way.1102
Example 7.4 An SA-SF-DFT calculation of singlet ground and excited states of ethylene.
$molecule 0 3 C C 1 B1 H 1 B2 2 A1 H 1 B3 2 A2 3 D1 H 2 B4 1 A3 3 D2 H 2 B5 1 A4 3 D3 B1 1.32808942 B2 1.08687297 B3 1.08687297 B4 1.08687297 B5 1.08687297 A1 121.62604150 A2 121.62604150 A3 121.62604150 A4 121.62604150 D1 180.00000000 D2 0.00000000 D3 180.00000000 $end $rem EXCHANGE bhhlyp BASIS cc-pvtz BASIS2 sto-3g UNRESTRICTED false CIS_N_ROOTS 5 SASF_RPA 1 CIS_TRIPLETS false $end