The multiconfigurational spin tensor electron propagator method (MCSTEP) is a single particle Green’s function (or electron propagator) method for determining the low-lying principal vertical ionization potentials (IPs) and electron affinities (EAs) of atoms and molecules, It was specifically designed to handle cases where the initial state has nondynamical correlation and/or is open shell. We have applied MCSTEP for the first time to triatomic molecules composed entirely of second row atoms. The two cases we present are O-3 and NO2-for the former nondynamical correlation is present in the ground (initial) state and for the latter the ground (initial) state is open shell. MCSTEP results are accurate compared to experiment and other forefront theoretical techniques.