TD-DFT is possible to calculate excited state that has different spin with the reference state. Here, we give some examples for such calculation.
Some important issues
Integral direct TDDFT
Since TDDFT is a capable to study medium to large molecules, so called integral-direct method is preferred in TD-DFT calculation. To apply integral-direct TDDFT, integral-direct SCF should be evoked that fully avoids to store electronic repulsive integrals in disk. Thus, molecules with several tens or even hundred of atoms are possible via TDDFT calcualiton.
Examples of spin-flip TDDFT
Here, we give some examples to perform spin-flip TDDFT calculation.
Spin-flip TDDFT to calculate triplet state of H2O molecule
We start from ground state of H2O. It is a close shell 1A1 state. We would like to calculate several triplet excited states using spin-flip TDDFT. Here is a input with comments. Notice: the words after # are comments. You may negelect them in your input.
$COMPASS Title H2O Molecule test run, cc-pvdz Basis cc-pvdz Geometry O 0.000000000 0.000000000 0.369372944 H 0.000000000 -0.783975899 -0.184686472 H 0.000000000 0.783975899 -0.184686472 End geometry Skeleton # You should not neglect this line. $END $XUANYUAN Direct # this line and the following line is important. Direct SCF is required. Schwarz $END $SCF RKS dft functional B3lyp $END # Spin flip up TDDFT calculation from RKS reference. $tddft imethod 1 # Starts from RKS calculation. 1 RKS, 2 UKS isf 1 # require spin flip up calculation idiag 1 itda 0 # TDDFT is evoked. iexit 4 # For each irreps, 4 states are required. $end # Spin flip up TDA calculation from RKS reference. $tddft imethod 1 # Starts from RKS calculation. 1 RKS, 2 UKS isf 1 # require spin flip up calculation idiag 1 itda 1 # TDA is evoked. iexit 4 # For each irreps, 4 states are required. $end
Tips: You may find this input in $BDFHOME/tests/input/h2o_spinflip_TDDFT.inp. TDDFT and TDA calculations are required. You may need only one of them.
Spin-flip TDA for open shell molecules
Spin-flip TDA can be used to calculate open shell systems. Here we calculate singlet of O2 molecule using spin-flip down TDA from triplet.
## Author, Bingbing Suo, Spin-flip TDA calculation of O2 molecule ### %echo " CHECKDATA AO-TDA ===" $COMPASS Title H10 Basis cc-pvdz Geometry O 0. 0. 0. O 0. 0. 1.5 END geometry skeleton Group D(2h) Check $END $XUANYUAN Direct Schwarz $END $SCF ROKS DFT B3LYP charge 0 spin 3 # triplet of O2 $END %echo "=== CHECKDATA Spin-flip down to get singlet. ====" $tddft imethod 2 isf -1 # spin flip down itda 1 idiag 1 iexit 4 ialda 2 $END %echo "=== CHECKDATA Spin-flip up to get quintet. ====" $tddft imethod 2 isf 1 # spin flip up itda 1 idiag 1 iexit 4 ialda 2 $END %echo "=== CHECKDATA Spin-flip up and down together ====" $tddft imethod 2 isf 3 # spin flip up and flip down itda 1 idiag 1 iexit 4 ialda 2 $END
Tips: You may find this input in $BDFHOME/tests/input/O2_spinflip_TDA.inp. Several spin-flip TDA calculations are required. You may need only one of them. Spin-flip U-TDDFT is not preferred.