Potential energy surfaces of quintet and singlet O4.

We present global ground-state potential energy surfaces for the quintet and singlet spin states of the O₄ system that are suitable for treating high-energy vibrational-rotational energy transfer and collision-induced dissociation in electronically adiabatic, spin-conserving O₂--O₂ collisions. The surfaces are based on MS-CASPT2/maug-cc-pVTZ electronic structure calculations with scaled external correlation. The active space has 16 electrons in 12 orbitals. The calculations cover nine kinds of geometrical arrangements corresponding to dissociative diatom-diatom collisions of O₂, geometries corresponding to O₃-O, geometries identified by running trajectories, and geometries along linear synchronous transit paths. The global ground-state potential energy surfaces were obtained by a manybody approach with an accurate O--O pairwise interaction and a fit of the many-body interaction to 12 684 electronic structure data points for the singlet and 10 543 electronic structure data points for the quintet. The many-body fit is based on permutationally invariant polynomials in terms of bond-order functions of the six interatomic distances; the bond-order functions are mixed exponential-Gaussian functions. [ABSTRACT FROM AUTHOR]