How to calculate T-T absorption energy of excited T1 state

I measured TA (Transient Absorption) spectroscopy of my compound and obtained the UV-Vis absorption data of the T₁ excited state. There is a peak that I suspect corresponds to a triplet–triplet absorption (T₁ → Tₙ transition), and I would like to confirm whether this peak indeed arises from a T–T absorption using Q-Chem calculations. Is there a way to do this?

Such calculations are possible with Q-Chem with a few methods—please refer to the Q-Chem User Manual (7.1.1 Overview‣ 7.1 General Excited-State Features ‣ Chapter 7 Open-Shell and Excited-State Methods ‣ Q-Chem 6.3 User’s Manual). If you have already tried such a calculation with Q-Chem and your method of choice but have further questions, please tell us how you are setting up the Q-Chem calculation by posting a sample input using the preformatted text icon “</>”.

I have already optimized the T₁ state of my complex using the input file below. Now, I would like to calculate the triplet–triplet (T–T) absorption energy and strength.


$rem
Job_type opt
Method B3LYP
Basis def2-SVP
DFT_D D3_BJ
Symmetry False
Max_SCF_cycles 3000
Geom_opt_max_cycles 3000
mem_total 2000000
mem_static 2000
CIS_N_Roots 20
CIS_Singlets False
CIS_Triplets True
Set_state_deriv 1
Max_CIS_cycles 3000
THRESH 14
GEOM_OPT_TOL_GRADIENT 100
GEOM_OPT_TOL_DISPLACEMENT 400
GEOM_OPT_TOL_ENERGY 33
XC_GRID 3
$end

@@@

$molecule
read
$end

$rem
Job_type freq
Method B3LYP
Basis def2-SVP
DFT_D D3_BJ
Symmetry False
Max_SCF_cycles 3000
mem_total 1000000
mem_static 2000
CIS_N_Roots 20
CIS_Singlets False
CIS_Triplets True
Set_State_Deriv 1
CPSCF_NSEG 2
$end
@@@

$molecule
read
$end

$rem
Job_type sp
Method B3LYP
Basis def2-TZVP
DFT_D D3_BJ
Solvent_method SMD
Symmetry False
Max_SCF_cycles 300
mem_total 200000
mem_static 4000
CIS_N_Roots 40
CIS_Singlets True
CIS_Triplets True
Max_CIS_cycles 300
Print_Orbitals True
Molden_Format True
State_Analysis True
CIS_Ampl_Anal True
CHELPG True
$end

$smx
Solvent THF
$end

You should be able to do this with the keyword STS_MOM, see below. The molecule in this input file is 4-(dimethylamino)benzonitrile (DMABN). Its triplet-triplet absorption spectrum is known from spectroscopic measurements - it shows absorption at around 550 nm (2.3 eV). The calculation predicts an intense T1 → T5 transition at 2.5 eV.

$molecule
0 1
C            -1.210008000    -1.233188000    -0.238719000
C             0.137726000    -1.240835000    -0.254259000
C             0.916264000    -0.009252000    -0.053578000
C             0.184187000     1.221130000     0.100746000
C            -1.167829000     1.256803000     0.127849000
C            -1.948305000     0.001647000    -0.010487000
N             2.295601000    -0.048042000    -0.108159000
C             3.041107000    -1.155993000     0.450034000
H             2.432578000    -2.065238000     0.480764000
H             3.363633000    -0.936485000     1.485953000
H             3.939712000    -1.355070000    -0.153268000
C             3.055470000     1.168846000    -0.267703000
H             4.067943000     0.918838000    -0.615050000
H             3.154022000     1.733662000     0.679206000
H             2.587794000     1.820797000    -1.016041000
H             0.731996000     2.155487000     0.230702000
H            -1.770985000    -2.152270000    -0.413282000
H             0.659424000    -2.172526000    -0.475250000
H            -1.704721000     2.194957000     0.263868000
C            -3.338804000     0.006170000     0.058697000
N            -4.507065000     0.014597000     0.116290000
$end

$rem
   scf_guess             sad
   scf_algorithm         diis_gdm
   MAX_SCF_CYCLES        1000
   JOBTYPE               sp
   UNRESTRICTED          false
   EXCHANGE              CAM-B3LYP
   BASIS                 def2-SVP
   XC_GRID               000099000590
   RPA                   false
   SCF_CONVERGENCE       8
   CIS_N_ROOTS           10
   CIS_CONVERGENCE       8
   max_cis_cycles        2000
   STS_MOM               TRUE
   CIS_singlets          False
   CIS_triplets          True
$end