Examples: O2
The SF-TDA/ALDA0 can be used to study triplet-singlet transitions in O2 through flip-down excitations.
$COMPASS Title H10 Basis cc-pvdz Geometry O 0. 0. 0. O 0. 0. 1.5 END geometry $END $XUANYUAN $END $SCF ROKS DFT B3LYP charge 0 spin 3 $END $TRAINT utddft orbi hforb $END $tddft imethod 2 isf -1 itda 1 idiag 1 ialda 2 $END
Results: The first one is Ms=0 component of the triplet state as indicated by D<S2>=0. The latter three are singlet states arising from excitations among open-shell orbitals [OO(ab) type excitations]. Other states are spin-contaminated as indicated by D<S2>=-1.
Imaginary excitation energies : 0 states
No. Pair ExSym ExEnergies f D<S^2> Dominant Excitations IPA Ova En-E1
1 Ag 1 B1g 0.0893 eV 0.0000 -0.0000 49.9% OO(ab): B2g( 1 )-> B2g( 1 ) 4.954 1.000 0.0000
2 Ag 2 B1g 1.0784 eV 0.0000 -1.9975 49.9% OO(ab): B3g( 1 )-> B3g( 1 ) 4.954 1.000 0.9892
3 B1g 1 Ag 1.0785 eV 0.0000 -1.9975 49.9% OO(ab): B2g( 1 )-> B3g( 1 ) 4.954 0.632 0.9892
4 B1g 2 Ag 2.0735 eV 0.0000 -1.9989 50.0% OO(ab): B3g( 1 )-> B2g( 1 ) 4.954 0.632 1.9843
5 B1u 1 Au 3.6453 eV 0.0000 -0.9868 49.2% CO(ab): B3u( 1 )-> B2g( 1 ) 8.272 0.967 3.5560
6 B1u 2 Au 4.5934 eV 0.0000 -0.9995 49.9% CO(ab): B2u( 1 )-> B3g( 1 ) 8.272 0.968 4.5041
7 Au 1 B1u 4.5934 eV 0.0000 -0.9995 49.9% CO(ab): B3u( 1 )-> B3g( 1 ) 8.272 0.612 4.5041
8 B3g 1 B2g 5.3092 eV 0.0000 -0.9887 98.8% CO(ab): Ag( 3 )-> B3g( 1 ) 8.714 0.633 5.2200
9 B2g 1 B3g 5.3092 eV 0.0000 -0.9887 98.8% CO(ab): Ag( 3 )-> B2g( 1 ) 8.714 0.633 5.2200
10 Au 2 B1u 5.4760 eV 0.0000 -0.9994 49.9% CO(ab): B2u( 1 )-> B2g( 1 ) 8.272 0.612 5.3868
11 B3u 1 B2u 5.4886 eV 0.0000 -0.9993 99.8% OV(ab): B2g( 1 )-> B1u( 3 ) 9.044 0.640 5.3993
Options
Using a UKS/ROKS reference in SCF, the input for spin-flip TDA (flip-up) reads:
$TDDFT IMETHOD 2 ISF 1 ... ialda 2 $END
The keyword ialda controls the spin-flip kernel when using GGA functionals in ground state calculations. The option "ialda=2" means a ALDA0 type approximation is used, which always gives numerical stable results.
Flip-down excitations can be calculated by choosing isf=-1.
$TDDFT IMETHOD 2 ISF -1 ... ialda 2 $END