Highly correlated single-reference studies of the O3 potential surface. I. Effects of high order excitations on the equilibrium structure and harmonic force field of ozone

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Abstract
The potential energy surface of ozone in the vicinity of the equilibrium geometry is investigated by single‐reference many‐body perturbation theory (MBPT) and coupled‐cluster (CC) methods. As expected from the known inadequacies of the independent‐particle picture of O3, analysis of the CCSDT‐1 wave function reveals considerable mixing between the [core⋅⋅⋅]4b226a211a22 and [core⋅⋅⋅]4b226a212b21 configurations. Smaller, but still significant, contributions come from other configurations involving redistribution of electrons within the out‐of‐plane π orbital framework. As expected, the equilibrium structure and harmonic force field computed at the SCF level of theory are in considerable error. When allowance is made for electron correlation effects, the discrepancies between theory and experiment for the equilibrium structure and totally symmetric force field are significantly reduced, and the MBPT(4), CCSD, CCSD+T(CCSD) and CCSDT‐1 results are in reasonable agreement with accepted values. Asymmetric stretching frequencies, however, are found to oscillate wildly [2373 cm−1 at MBPT(2), 1547 cm−1 at MBPT(4), and 680 cm−1 at CCSDT‐1] about the experimental value of 1089 cm−1, suggesting that the Cs subspace of the potential surface poses a great challenge to theory. A possible cause of this behavior is discussed, along with implications for future studies of the global O3 potential surface.