Accurate vertical ionization potentials of CH2, NH2, and H2O using the multiconfigurational spin tensor electron propagator method (MCSTEP)

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The multiconfigurational spin tensor electron propagator method (MCSTEP) is used to determine accurate low-lying vertical principal ionization potentials (IPs) for the free radicals CH2 and NH2 and the closed shell molecule H2O. In previous MCSTEP calculations we had compared MCSTEP IPs with full configuration interaction (FCI) values for CH2 using a double zeta with polarization (DZP) basis set. Those MCSTEP results were in very good agreement with ΔFCI. An alternative complete active space (CAS) choice used in this paper gives even better agreement—differing from ΔFCI values by 0.03 eV and 0.01 eV for the lowest two vertical principal IPs from the 3B1 neutral ground state. These MCSTEP results are extended by also using pVDZ, pVTZ, and pVQZ basis sets at both the previous (FCI) geometry and the experimental geometry. Choosing the CAS in the same manner for NH2 yields very good principal vertical IPs compared with experiment. Comparisons are again made using pVDZ, pVTZ, and pVQZ basis sets. Low-lying principal vertical MCSTEP IPs are obtained for H2O and compared with recent ΔFCI values using the same basis set and geometry as the FCI calculations. Low-lying principal vertical MCSTEP IPs are also obtained using pVDZ, pVTZ, and pVQZ basis sets at the experimental geometry.