Iodine in the Atmosphere I: Computational Benchmark and Dimer Formation of Oxy-acids and Oxides

The contribution of iodine-containing compounds to atmospheric new particle formation is still not fully understood, but iodic acid and iodous acid are thought to be significant contributors. While several quantum chemical studies have been carried out on clusters containing iodine, there is no comprehensive benchmark study quantifying the accuracy of the applied methods. Here, we present the first paper in a series that investigate the role iodine species in atmospheric cluster formation. In this work, we have studied the iodic and iodous acid monomers, the iodine tetroxide and iodine pentoxide monomers, and their dimer formation with common atmospheric precursors. We have tested the accuracy of commonly applied methods for calculating the geome- try of the monomers, thermal corrections of monomers and dimers, the contribution of spin-orbit coupling to monomers and dimers, and finally the accuracy of the electronic energy correction calculated at different levels of theory. We find that optimizing the structures either at the ωB97X-D3BJ/aug-cc-pVTZ-PP or the M06-2X/aug-cc-pVTZ-PP level achieves the best thermal contribution to the binding free energy. The electronic energy correction can then be calculated at the ZORA-DLPNO-CCSD(T0) level with the SARC-ZORA-TZVPP basis for iodine and ma-ZORA-def2-TZVPP for non-iodine atoms. This combination of methods yields results in excellent agreement with fully relativistic calculations at a comparatively low computational cost. We applied this methodology to calculate binding free energies of the iodine-containing dimer clusters, where we confirm the qualitative trends observed in previous studies. However, we identify that previous studies overestimate the stability of the clusters by several kcal/mol due to the neglect of relativistic effects. This means that their contribution to the currently studied nucleation pathways of new particle formation is likely overestimated.