Multireference quantum Monte Carlo study of the O4 molecule

Fixed-node diffusion quantum Monte Carlo (FN-DMC) calculations are performed to obtain the most accurate dissociation barrier and heat of formation with respect to dissociation into molecular oxygen for the chemically bound tetraoxygen molecule. Multireference trial wave functions were used and built from truncated CASSCF(16,12) through a weight-consistent scheme allowing to control the fixed-node error. Results are compared with the previous ab initio benchmark Complete Active Space SCF Averaged Coupled Pair Functional/aug-cc-pVQZ (CASSCF-ACPF/AVQZ) results. The FN-DMC barriers to dissociation and heat of formation obtained are 11.6+/-1.6 kcal/mol and 98.5+/-1.9 kcal/mol, respectively. These thermochemical energies should be taken as the theoretical references when discussing the relevance of tetraoxygen in a variety of experiments and atmospheric chemical processes.