Ab initio calculations on systems involving singlet molecular oxygen (O2 (1g)) are challenging due to signicant multi-reference character arising from the degeneracy of the HOMO and LUMO orbitals in singlet oxygen. Here we investigate the stragegy of bypassing singlet oxygen’s multi-reference character by simply adding the experimen- tally determined singlet/triplet splitting (22.5 kcal/mol) to the triplet ground state of molecular oxygen. This method is tested by calculating rate constants for the reac- tions of singlet molecular oxygen with furan, 2-methylfuran, 2,5-dimethylfuran, pyrrole, 2-methylpyrrole, 2,5-dimethylpyrrole, and cyclopentadiene. The calculated rate con- stants are within a factor of 15 compared to experimentally determined rate constants. The results show that energy renement at the CCSD(T)-F12 level of theory is cru- cial to achieving accurate results. The reasonable agreement with experimental values validates the bypassing approach which can be used for other systems involving the 1,4-cyclo-addition of singlet oxygen. 2