7.2 Uncorrelated Wave Function Methods

7.2.5 Spin-Adapted Spin-Flip CIS

Spin-Adapted Spin-Flip CIS (SA-SF-CIS)1040 is a spin-complete extension of the spin-flip single excitation configuration interaction (SF-CIS) method.498 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,1040 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.1040

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