Ground and low‐lying excited C2v states of FeO2—A challenge to computational methods

DFT (BPW91 and B3PW91), CCSD, CCSD(T), and MP2 geometry optimizations were performed on the lowest A₁, A₂, B₁, and B₂ singlet to septet states (a total of 16 states) of FeO₂, using in most cases the 6‐311+G(3df) basis set. With few exceptions, the different methods led to similar geometries. The lowest state is ³B₁ or ⁵B₂, depending on the method used. The quality of the various computational methods was tested by a comparison of vertical excitation energies with corresponding high‐level MRCI values. The best agreement with MRCI was found for BPW91 and B3PW91, with average deviations of about 0.2 eV. MRCI calculations were carried to extremely low configuration selection thresholds (0.025 μh) in order to find the lowest state of FeO₂. The full CI estimate (including Davidson correction and energy extrapolation to 0 μh) gave ³A₁ as the lowest state, followed by ⁵B₂ (0.03 eV) and ³B₁ (0.06 eV). Both ³B₁ and ³A₁ have geometries and vibrational frequencies consistent with the observed IR spectrum. The calculated adiabatic electron affinity of FeO₂ (from ³B₁ of FeO₂ to ⁴B₂ of FeO) is 2.46 eV with CCSD(T), and 2.22 eV with BPW91, compared with the experimental value of 2.36 eV. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009