Dear wsoulie,
I learned from the above email that you are calculating g_lum. I think i can get some help from you. I am trying the 18 then 5 option. I am providing the td-dft output file (done for the triplet 1 state ), now after entering 5 , it is asking for the Multiplicity 1 or 3 , which one should i choose and how should i proceed and how will I get the data about my t1 to s0 transition dipole moments.

Thank you

Assume you are a Gaussian user and you used 50-50 option in TD keyword, then singlet and triplet excited states are all calculated, so Multiwfn asks you to choose the spin multiplicity of the excited state of interest. As you are only interested in T1, you should choose spin multiplicity of 3 in Multiwfn.

Thank you

Frankly speaking, I am not sure if the statement in ESI of (Garner & Corminboeuf, PCCP 2023, ESI) is correct, perhaps it is a misunderstanding. You may contact the paper author or Gaussian's official custom service to further confirm this point.

I wish to use Multiwfn to compute the magnetic and electric transition dipole moments (MTDM and ETDM) from my Gaussian output, using option 18 → 5.

However, a question remains regarding units: Multiwfn labels both quantities as "atomic units", but I am unsure whether the MTDM is given in Bohr magnetons (μ_B) or in e·a₀. These are not the same unit — they differ by a factor of α/2, where α is the fine structure constant.
My goal is to compute the luminescence dissymmetry factor using:

g_lum = 4·|μ|·|m|·cos(θ) / (|μ|² + |m|²)

For this formula to be dimensionally consistent, both |μ| and |m| must be expressed in the same units. So the question is: should I convert the MTDM from μ_B to e·a₀ by multiplying by α/2?

I found one reference explicitly documenting Gaussian's convention (Garner & Corminboeuf, PCCP 2023, ESI), which states that magnetic transition dipole moments from Gaussian are given in Bohr magnetons — implying that the conversion is necessary. However, I have not been able to confirm whether Multiwfn preserves this convention or internally converts to e·a₀ before printing.

Thank you for your help.