It looks, from the sodium example in the manual, that one calculates the two states to be spin-orbit coupled, and then performs a CITYP=GUGA RUNTYP=SPINORBT calculation. Note that I have never done a CI calculation with Firefly. Do I have to become a CI expert before attempting a spin-orbit calculation?
Also, why are ROHF and GVB used to find the two states to be coupled instead of UHF? And what do you do if you have molecules or radicals that would seem to be solidly singlet or solidly doublet based on formula? (I am interested for now in SnH, SnH2, SnH3, and SnH4, and their successive bond energies). I don't know what orbital L numbers are for multi-atom species.