I am trying to calculate PES for Molecular Oxygen and Atomic Nitrogen Exchange reaction. This is also known as Zeldowich reaction. I found that for asingle reactions, there are many PES's which I believe are for different electronic configurations of the NO2 PES. I found that the multiplicity can be 2 or 4. Also I find different surface being named as A' and A" in the literature. I would like to what exactly are these and how can I compute these in Gaussian?