Theoretical studies of the ozone molecule. I. A b i n i t i o MCSCF/CI potential energy surfaces for the X 1A1 and a 2B2 states

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Abstract
The potential energy surfaces for the X 1A1 and first-excited a 3B2 states of ozone are surveyed with accurate ab initio electronic structure calculations. An efficient technique for using the multiconfiguration self-consistent field (MCSCF) method for polyatomics has been established for these computations, and is described in some detail. The method involves a geometry-dependent flow of configurations between the MCSCF wave function and a small configuration interaction (CI) wave function. The configurations are constructed in a set of 8 core, 7 valence, and 4 ’’virtual’’ MCSCF orbitals, which are expanded in a double-zeta plus polarization one-electron basis. The vertical and adiabatic excitation energies, uncorrected for zero-point energies, are presently obtained as 1.21 and 0.74 eV, respectively. These results are in agreement with other quantitative theoretical studies which are now available for comparison. Furthermore, the present results indicate that O3(a 3B2) has an O2–O binding energy Eb=0.4 eV and that there is no barrier relative to its adiabatic asymptote, O(3P)+O2(X 3Σ−g). These energies are estimated to be accurate to within 0.2 eV. Thus, we conclude that O3(a 3B2) is a bound state of ozone. Similar MCSCF/CI calculations are reported for the ring state. The implications of this work for ozone chemistry are discussed. The existence of a second bound state, relative to the ground state atom–diatomic asymptote, may have important consequences with regard to the interpretation and modeling of ozone formation from O(3P) and O2(X 3Σ−g): the a 3B2 state, as well as the X 1A1 state, may be involved.