Illustrations des systèmes modélisés pour (a) distribution de molécules d’eau sur la surface de l’aérosol organique (inset : modèle QM/MM pour le domaine HO2-phase organique) ; (b) modèle imbriqué pour les halogénures dans des gouttelettes d’eau (rouge : densités électroniques DFT pour l’eau ; bleu : halogénure) ; (c) modèle structurel pour l’interaction chlorure-glace amorphe (d) snapshot de la simulation de l’ion iodate solvaté.
Aerosols are small solid particles or liquid droplets in excess- pension. They originate from natural sources (volatile organic compounds emitted by biomass, sea salts, etc.) or by human action (soot from incomplete fuel com- bustion).
These aerosols play a key role as substrates on which species present in the atmosphere can attach, react contributing to their dispersion around the globe, including radionuclides such as iodine. These phenomena involve multiple physico-chemical processes which, to be understood at the molecular scale, require the use of theoretical models combining different approaches.
Our team uses and develops these models and the associated simulation codes. Our simulations aim to describe electrical and/or magnetic, and/or thermodynamic properties using a solute/solvent approach, including environmental and temperature effects (averaging results over a range of configurations). In particular, we can simulate: (a) the reactive uptake of HO2 on organic aerosols, through the combination of classical molecular dynamics (CMD) simulations and non-relativistic ab initio QM/MM calculations; (b) XPS spectra of the microhydrated iodide ion, and the evolution of halide valence ionization spectra with droplet size, by combining various methods developed by the team (polarizable force fields for CMD simulations, and the EOM-IP-CCSD-in-DFT QM/QM relativistic quantum nesting method); (c) XPS spectra of HCl and chloride ion at air/ice interfaces coupling relativistic EOM-IP-CCSD-in-DFT; and (d) solvation of iodate ion (IO3-), using Car-Parrinello MD (CPMD) simulations in the absence of conventional polarizable force fields.
The very large volume of simulations required for these example studies has made the use of national computing centers essential to achieve our objectives.