I am doing DFT calculations on a series of compounds, but I have two important questions.
1- How can I calculate the spin multiplicity for a large molecule with -1 and -2 charge.
2- To calculate the redox potential for the transfer of the second electron, am I allowed to do the calculations in the gas phase for the anion with charge -1 to obtain the free energy change in the gas phase? (And then the calculations in the solvent state for the anion with charge -1 and -2)? Or is only the free energy change calculations valid for the neutral species in the gas phase?
1) The spin multiplicity is an input parameter at the beginning, not a result at the end, so if you don't know it from intuition, you'll have to try what's more stable. Please be aware that DFT is a single reference method, so even if it converges, there's no guarantee that it will be accurate.
2) Redox potentials are highly non-trivial (as is everything with DFT and solvents); here is a guideline:
https://pubs.rsc.org/en/content/articlelanding/2016/cp/c5cp06638g
Maybe someone with more expertise than me can weigh in on the second part as well.