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 A(1), A(2), B-1, and B-2, singlet to septet skates (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 B-3(1) or B-5(2), 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. he 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 mu h) in order to find the lowest state of FeO2. The full CI estimate (including Davidson correction and energy extrapolation to 0 mu h) gave (3)A(1) as the lowest state, followed by B-5(2) (0.03 eV) and B-3(1) (0.06 eV). Both B-3(1) and (3)A(1) have geometries and vibrational frequencies consistent with the observed IR spectrum. The calculated adiabatic electron affinity of FeO2 (from B-3(1) of FeO2 to B-4(2) of FeO2-) is 2.46 eV with CCSD(T), and 2.22 eV with BPW91, compared with the experimental value of 2.36 eV. (C) 2008 Wiley Periodicals, Inc. Int J Quantum Chem 109: 549-558, 2009