# S1-T1 energy difference in Gaussian

I would like to calculate the energy difference between the first excited singlet and the first excited triplet of a given molecule in solvent, on Gaussian. In this publication, they state that using TDA-DFT can be a good level of theory for this type of calculation. For this reason, I have built two inputs. To calculate the singlet:

# opt M062X/6-311g* TDA=(singlet,root=1) scrf=(solvent=dichloromethane) int=ultrafine Nosymm


and to calculate the triplet

# opt M062X/6-311g* TDA=(triplet,root=1) scrf=(solvent=dichloromethane) int=ultrafine Nosymm


After running these calculations, I obtain two outputs that contain, for the singlet:

    Excited State   1:   Singlet-?Sym    3.0913 eV  401.07 nm  f=0.7556  <S**2>=0.000
110 ->111         0.67231
This state for optimization and/or second-order correction.
Total Energy, E(CIS/TDA) =  -1315.38267036
Copying the excited state density for this state as the 1-particle RhoCI density.

Excited state symmetry could not be determined.
Excited State   2:  Singlet-?Sym    3.6009 eV  344.32 nm  f=0.0405  <S**2>=0.000
106 ->111         0.14329
107 ->111        -0.15242
108 ->111         0.32533
110 ->112         0.51705
110 ->113         0.18032

Excited state symmetry could not be determined.
Excited State   3:  Singlet-?Sym    3.9376 eV  314.87 nm  f=0.1058  <S**2>=0.000
106 ->111        -0.22741
106 ->112         0.10392
108 ->111         0.11810
108 ->115         0.11499
109 ->111         0.50685
110 ->112         0.12586
110 ->113        -0.28556


and for the triplet:

Excited State   1:   Triplet-?Sym    2.1716 eV  570.93 nm  f=0.0000  <S**2>=2.000
109 ->111        -0.10733
110 ->111         0.66438
This state for optimization and/or second-order correction.
Total Energy, E(CIS/TDA) =  -1315.41646875
Copying the excited state density for this state as the 1-particle RhoCI density.

Excited state symmetry could not be determined.
Excited State   2:  Triplet-?Sym    2.9918 eV  414.41 nm  f=0.0000  <S**2>=2.000
109 ->111         0.34352
109 ->113        -0.11747
110 ->111         0.13157
110 ->112         0.55107

Excited state symmetry could not be determined.
Excited State   3:  Triplet-?Sym    3.3346 eV  371.81 nm  f=0.0000  <S**2>=2.000
107 ->111         0.14468
108 ->111        -0.25678
109 ->111        -0.34944
109 ->112        -0.21840
110 ->112         0.22031
110 ->113         0.37233


I am a little confused by the output itself. Is the energy that one should consider for the difference in energy the one reported after 'Total energy' (for example, -1315.38267036 for the singlet) or the one reported after the 'Excited state 1' (3.0913eV for the singlet)?

• Just a heads-up, Gaussian includes a TDA=(50-50) (and also TD=(50-50)) option to calculate the singlets and triples at the same time, which is easier and should save time. Mar 2 at 16:39