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.

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.

$COMPASS
Title
 H10
Basis
 cc-pvdz
Geometry
 O 0. 0. 0.
 O 0. 0. 1.5
END geometry
skeleton
$END

$XUANYUAN
Direct
Schwarz
$END

$SCF
ROKS
DFT
B3LYP
charge
0
spin
3         # triplet  of O2 
$END

# Spin-flip down to get singlet.
$tddft
imethod
2
isf
-1   # spin flip down
itda
1
idiag
1
$END

# Spin-flip up to get quintet.
$tddft
imethod
2
isf
1     # spin flip up
itda
1
idiag
1
$END

# Spin flip up and flip down together.
$tddft
imethod
2
isf
3       # spin flip up and flip down
itda
1
idiag
1
$END