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Is there free software that can calculate the fluorescence emission spectra of organic molecules? Let's say, for example, rubrene

There is a bunch of quantum free quantum chemistry software: https://en.wikipedia.org/wiki/List_of_quantum_chemistry_and_solid-state_physics_software However, which one would be the most suitable for this purpose? Could this be done on a reasonable up-to-date laptop? Of course, computing times will depend on the number of atoms in the molecule and the desired precision. What would be approximate calculation times? In the order of hours, days or weeks? I can imagine that this is most probably not a simple "draw molecule and push calculate-button"-task, but any hints where to further dig into are appreciated.

Addition after the question was put on hold: This question certainly can fill a semester of a lecture on quantum or computational chemistry. In that sense, yes, it is broad.

However, instead of detailed instructions or input scripts, I was hoping for some rough guidance about the essential steps and the available software which can be recommended for that purpose (and equally important: which software can be excluded or cannot be recommended to avoid investing a lot of time and find out later that there are much better options). And of course, whether you need a super-computer or whether you can do it on a laptop within a reasonable amount of time.

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  • $\begingroup$ As a start you could assume harmonic potentials and with the displacement between excited and ground state potentials calculate the Franck-Condon factors for displaced and different frequency oscillators, see Chem. Phys. Letts, v 18 p 481, 1973 for the equations. V simple calculation. Most aromatics only have a couple of FC active modes that dominate the spectrum which is just as well as there are 3N-6 modes and using all combinations/overtones in a full calculation involves a truly vast number of pairs of levels. $\endgroup$
    – porphyrin
    Commented Jul 19, 2019 at 11:50
  • $\begingroup$ Thank you for the reference, I will check. Although, I my goal is not to implement the equations myself. Therefore, I'm hoping for established software to take the geometry of the molecule (i.e. position and type of atoms) as input, which I guess need to be calculated first. So, in short, ideally SMILES (or others) code in --> spectrum out. $\endgroup$
    – theozh
    Commented Jul 19, 2019 at 12:08
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    $\begingroup$ You could use the Avogadro software to create input files from SMILES for a number of different codes (Gaussian, Orca, Psi4, ...). But you will still have to select the actual method and basis set and tell it run a geometry optimization and frequency analysis. This would give you an IR/Raman spectrum. Fluorescence is more complicated than that, as it includes couplings between different electronic and vibrational excited states. $\endgroup$
    – Feodoran
    Commented Jul 19, 2019 at 14:20
  • $\begingroup$ Could you edit your question to include the additional information from your comment. Your question has been flagged as too broad. $\endgroup$
    – Karsten ♦
    Commented Jul 19, 2019 at 17:03
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    $\begingroup$ What I was trying to say: you can get something half-way black box for IR spectra. But since fluorescence is more complicated than that ... I don't think what you are looking for exists. $\endgroup$
    – Feodoran
    Commented Jul 19, 2019 at 20:59

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As you mentioned, Gaussian can do it but is proprietary.

I believe ezSpectrum (https://github.com/iopenshell/ezSpectrum) might be able to do what you are looking at but I have not used it myself so far.

There used to be a program available called FCClasses which would run the Franck-Condon computations for you based on (I think) Gaussian output, but it seems like the author took it down, I can only find a front-end to it right now: https://github.com/jcerezochem/fcc_tools The FCClasses homepage is mentioned as http://www.pi.iccom.cnr.it/fcclasses on that page but, as mentioned above, no longer works.

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