I am using Gaussian 09. I need to calculate delta(EST) energy gap for TADF calculation (for organic molecules).
This is assuming fluorescence occurs from the S1 state.
1. Optimize and do frequency calculation of the ground state geometry of the molecule. Use charge=0, multiplicity=1.
2.Optimize and do frequency calculation S1 (first excited state geometry (opt TD with root=1, singlet,nstate=6). This will give fluorescence.
3. Optimize and do frequency calculation ground state geometry with charge=0 and spin multiplicity=3. This will give T1 geometry (without the TD keyword). Use unrestricted spin option.
4.Do TDDFT absorption with this triplet (T1) geometry, charge =0, multiplicity =3 to get phosphorescence (energy of T1 state ). Use unrestricted spin option.
The energy difference between 2 and 4 gives delta(S1-T1) - singlet-triplet gap.
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