Accurate Yet Affordable: An Integrated Tool for the Simulation of Criegee Intermediates via Pisa Composite Schemes and Localized Corrections.

Criegee intermediates (CIs) are pivotal reactive species in atmospheric chemistry, playing a central role in alkene ozonolysis and secondary organic aerosol formation. However, their transient nature and unconventional electronic structure pose a serious challenge to both experimental characterization and theoretical modeling. In this work, we present an integrated and cost-effective computational platform for the accurate simulation of the structure and rotational constants of CIs, based on the Pisa Composite Schemes (PCSs) and localized correction strategies. Geometry optimizations on composite (including multilayer) potential energy surfaces are enabled by a newly developed interface that combines the strengths of different quantum chemistry codes. Benchmarking against the semiexperimental equilibrium structure of formaldehyde oxide allows the derivation of transferable corrections that extend the spectroscopic accuracy to larger Criegee systems. We also introduce a two-layer ONIOM scheme in which a DFT description of the full molecule is refined by higher-level corrections localized on the carbonyl oxide moiety. This approach, validated on cyclohexanone oxide, recovers high-level accuracy at a fraction of the computational cost of full high-level optimizations. Overall, our method provides a robust and automated framework for the spectroscopic characterization of Criegee intermediates, thereby broadening the reach of computational spectroscopy in atmospheric chemistry.