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

Area of Science:

• Atmospheric Chemistry
• Computational Chemistry
• Quantum Chemistry

Background:

• Recent increases in iodine emissions have spurred interest in atmospheric iodine chemistry.
• Iodine oxides and oxyacids are key precursors to atmospheric particle formation.
• Understanding the simplest iodine oxides, like I2O2, is crucial for atmospheric models.

Purpose of the Study:

• To investigate the four isomers of I2O2.
• To determine the lowest energy isomer and relative isomer ordering.
• To assess the performance of various Density Functional Theory (DFT) functionals for I2O2 calculations.

Main Methods:

• High-level ab initio calculations using CCSD(T)/aug-cc-pwCVTZ-PP.
• Extensive basis sets up to quintuple zeta and coupled-cluster methods up to CCSDT(Q).
• Focal point approach for complete basis set limit relative energies.
• Benchmarking 30 DFT functionals against CCSD(T) results.

Main Results:

• The IIO2 isomer is identified as the lowest energy structure.
• Relative energies of I2O2 isomers are clarified, resolving previous discrepancies.
• B2GP-PLYP demonstrated superior performance among tested DFT functionals.
• G3LYP and TPSSh offer affordable alternatives with comparable accuracy.

Conclusions:

• The IIO2 isomer is the most stable form of I2O2.
• The study provides accurate energetic data for I2O2 isomers.
• DFT functional performance varies, with B2GP-PLYP recommended for I2O2 studies.
• DFT performance hierarchy is not consistently followed, especially excluding double hybrids.