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7.2 Uncorrelated Wave Function Methods

7.2.6 Spin-Adapted Spin-Flip CIS

(February 4, 2022)

Spin-Adapted Spin-Flip CIS (SA-SF-CIS)1273 is a spin-complete extension of the spin-flip single excitation configuration interaction (SF-CIS) method.601 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,1273 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.1273 As with other SF-TDDFT methods,1274 the BH&HLYP functional has become something of a de facto standard choice.459, Herbert:2022

Example 7.4  An SA-SF-DFT calculation of singlet ground and excited states of ethylene.

0 3
 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

   EXCHANGE       bhhlyp
   BASIS          cc-pvtz
   BASIS2         sto-3g
   CIS_N_ROOTS    5
   SASF_RPA       1
   CIS_TRIPLETS   false

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