I2O2: High level theoretical studies and a benchmark of DFT functionals

Increasing iodine emissions over recent years have led to new interest in atmospheric iodine chemistry. The primary species involved include iodine oxides and oxyacids-precursors to atmospheric particle formation. Four isomers of the simplest iodine oxide belonging to the I2Ox family, I2O2, are investigated here. Geometries and anharmonic frequencies are obtained at the CCSD(T)/aug-cc-pwCVTZ-PP level of theory. Stationary points were treated at much higher levels of theory, with basis sets up to quintuple zeta and methods up to CCSDT(Q). The focal point approach is used to report relative energies at the complete basis set limit. Previous studies disagree concerning the isomer lying lowest in energy and the relative ordering of the isomers in general. The IIO2 isomer is found here to lie lowest in energy relative to IO + IO. The performance of 30 different density functional theory (DFT) functionals is also assessed in predicting geometries, vibrational frequencies, and energies relative to CCSD(T) given the popularity of DFT methods in the literature. The B2GP-PLYP functional is found to be the overall best in the present benchmark. The G3LYP and TPSSh functionals perform similarly to each other and are more affordable alternatives to double hybrids. Excluding the double hybrid functionals, the performance of the functionals included in this study does not follow the hierarchy of the DFT ladder.