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wham09

Member

Registered: 2025-09-19

Posts: 66

Large gap between two methods used to measure S0-T1 excitation energy

Dear Prof. Lu,

I am studying a S0 -> T1 excitation of an organic bimolecular encounter complex.

I initially used the following method:

1. S0 state calculation and save non-equilibrium solvation
%chk=S0.chk
# wB97XD/def2TZVP scrf=(read, solvent=dimethylsulfoxide)

0 1
[geometry]

NonEq=write

2. T1 state calculation with external iteration
%oldchk=S0.chk
%chk=T1.chk
# wB97XD/def2TZVP scrf=(externaliteration, read, solvent=dimethylsulfoxide) geom=check guess=read TD(nstates=5, triplets)

0 1

NonEq=read

Then I obtained the energy difference between #1 and #2.

Later, I tested the following method. Using the S0.chk from above:

%oldchk=S0.chk
%chk=T1-DFT.chk
# wB97XD/def2TZVP scrf=(externaliteration, read, solvent=dimethylsulfoxide) geom=check guess=read

0 3

NonEq=read

Then I obtained the energy difference between #1 and this calculation.

From the former method using S0 (SCF) vs. T1 (TD, external iteration), I got 335.2 nm wavelength, while from the latter method using S0 (SCF) vs. T1 (SCF, external iteration), I got 279.4 nm.
I checked the electronic structures of the two T1 states and they were similar (both the same valence-excited state). However, the excitation energy difference is quite large.
Which method would you say is more reliable? I assumed that for this case, it is okay to use external iteration for SCF also because the spin state is set different.

Last edited by wham09 (2026-07-06 07:16:56)

sobereva

Tian Lu (Multiwfn developer)

From: Athens, Greece

Registered: 2017-09-11

Posts: 2,267

Website

Re: Large gap between two methods used to measure S0-T1 excitation energy

I suggest first checking difference between the two methods of evaluating E_ST gap in gas phase. The difference in excitation energy calculated by delta-SCF and TDDFT is not negliglble, may be as large as ~0.2 eV or even more. In principle, the gap calculated using delta-SCF is more reasonable, because it fully consideres orbital relaxation of T1, and there is no potential issue of triplet instability.
If in the gas phase the difference is not large, then you can further focus on checking the treatment of solvation effect.

wham09

Member

Registered: 2025-09-19

Posts: 66

Re: Large gap between two methods used to measure S0-T1 excitation energy

Thank you for the reply.
Indeed I found out that the E_ST gaps in gas-phase and solution-phase (external iteration) are:

delta-SCF method: 4.39 eV (gas-phase) / 4.44 eV (solution-phase / external iteration)
TDDFT method: 3.69 eV (gas-phase) / 3.70 eV (solution-phase / external iteration)

So the gas-phase difference seems to be the dominant factor. Would you then say that the delta-SCF method is probably more reliable?

Also I have two additional questions if you don't mind:

1. For this specific purpose of the delta-SCF method for vertical E_ST gap, am I correct that external iteration is meaningful, unlike for normal SCF?

2. Can I do the state-specific external iteration method for TDDFT calculation in ORCA? I can't find anything similar that is mentioned in the manual or tutorial.
If the answer is no, would you say that ORCA is fundamentally less powerful than G16 for calculation of excited states accompanying significant electronic structure reorganization, CT states for example, in highly polar solvents?