Reassessing spin-coupled (full generalized valence bond) descriptions of ozone using three-center bond indices

Domain-averaged Fermi hole analysis is carried out for the ground state of O3 at its equilibrium geometry using a complete-active-space self-consistent field CASSCF(18,14) wavefunction, based on a slightly expanded full-valence active space. This initial analysis is augmented with an examination of the corresponding localized natural orbitals (LNOs) and of the numerical values obtained with a new improved definition of three-center bond indices for correlated singlet systems. Much the same pattern of LNOs is observed when using instead a subsequent internally-contracted multiconfiguration-reference configuration interaction construction, which also provides very similar values for the three-center bond indices. This gives us confidence to use such bond indices, alongside relative energies and the electric dipole moment, to assess the relative merits of various combinations of spin-coupled (full generalized valence bond) components with ten active electrons: four pi, four sigma bonding and the two nonbonding sigma electrons associated with the central O atom. These multi-component valence bond descriptions were generated either with or without subsequent orbital reoptimization. The description of the pi system which emerges from all of our analysis conforms to a standard model of three-center four-electron pi bonding that incorporates a nontrivial degree of (partial) diradical character. Whereas certain combinations of ten-electron spin-coupled components can faithfully reproduce such a picture, none of the individual rival components appears to have sufficient flexibility on its own.