CCSDT study of the fluoroperoxyl radical, FOO

The FOO radical was studied at the coupled-cluster (CC) theoretical level, employing both the perturbative calculation of the contribution of triple excitations, CCSD(T), and the exact evaluation, CCSDT. Two solutions were found, one of them with a large spin contamination. It is shown that spin contamination is a problem at the CCSD(T) level, while it is largely inmaterial at the complete CCSDT level. The full CCSDT level affords an FO distance of 1.632 ± 0.005 Å in reasonable agreement to experiment. The enthalpy of formation at the uncontaminated CCSDT level, ΔfH0298(FOO)=6.5±1 kcal/mol, is also in very good agreement with the experimental value of 6.24 ± 0.5 kcal/mol. Contrary to previous studies, CCSDT performs better than CCSD(T) in the calculation of the properties of this radical.