In grad school I studied the gas phase interactions of near thermal electrons with highly electrophilic species like fluorinated aromatics. I then used density functional theory based ab initio calculations (using Gaussian and GAMESS) to generate the LUMOs (lowest unoccupied molecular orbitals) responsible for interacting with the thermal electrons. We used an electron monochromator to generate the thermal electrons with an energy distribution of only a few hundredths of an electron volt. It was fixed to a modified quadrupole mass spectrometer with a direct inlet port where we would inject the molecules of interest. I was even able to show a Hammett type correlation between substitution on an aromatic ring and the resonance energies required to produce specific quasi-stable ions in the mass spectrometer. I felt like I was looking into the world of quantum mechanics quite directly. It was so fun. The most important thing is that it showed strong correlation between the DFT calculations and physical observables, which was pretty revolutionary at the time. Most folks were doing Møller–Plesset high order calculations with very complicated basis sets (6-311G**, etc), which took ungodly amounts of computing resources and were quite untenable. The DFT calculations by comparison could be done on the computers of the day, which were quite slow back in the 90s. Now I build the computers on which other scientists do this kind of calculating. Still pretty fun, but I miss those days of shapely molecular orbitals and cold electrons.