How to calculate molecular emission spectra? 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.
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.