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.