Large multiconfiguration self-consistent-field wave functions for the ozone molecule

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Abstract
The electronic structure of the ozone molecule is of particular interest in light of Goddard’s characterization of the ground state as a biradical. Rigorously optimized multiconfiguration self‐consistent‐field (MCSCF) wave functions of varying size have been determined here for ozone via newly developed techniques utilizing the unitary group approach. The largest of these ab initio MCSCF wave functions includes 13 413 configurations, i.e., all singly‐ and doubly excited configurations relative to the two reference configurations required for the biradical description of ozone. The convergence of the MCSCF procedures is discussed, as well as the structure of the MCSCF wave functions, and the effectiveness of different orbital transformations. There is a significant energy difference (0.034 hartrees) between the MCSCF wave functions involving one and two reference configurations. This gives emphasis to the fact that orbital optimization alone cannot compensate for the exclusion from the wave function of important classes of configurations. A simple test for the determination of the fraction biradical character of systems such as ozone suggests 23% biradical character for 03 at its equilibrium geometry.