PCS2-Based Schemes for Geometry Optimization and Frequency Calculations in Criegee Intermediate-Water Reactions.

Area of Science:

• Atmospheric Chemistry
• Quantum Chemistry
• Computational Modeling

Background:

• Criegee intermediates are key species in atmospheric chemistry.
• Their reactions with water are vital for air quality modeling.
• Unusual electronic structures challenge standard computational methods.

Purpose of the Study:

• To benchmark new computational approaches for Criegee intermediate reactions with water.
• To validate the Pisa Composite Scheme (PCS2) for accuracy and efficiency.
• To assess the impact of low-level method selection on reaction rate predictions.

Main Methods:

• Dual-level composite methods combining affordable geometry optimizations with high-level energy calculations (W2X).
• Explicitly correlated coupled-cluster theory [CCSD(T)-F12b] and its analytical-gradient variant.
• Automated workflow for efficient exploration of computational parameter space.

Main Results:

• PCS2 and its variant accurately reproduce geometries and activation enthalpies.
• Basis set convergence achieved at the double-ζ F12 level.
• Low-level method choice significantly impacts rate constants (30-60% errors).

Conclusions:

• Dual-level PCS-based approaches are reliable for atmospheric chemistry applications.
• Careful selection of low-level methods is critical for accurate kinetic predictions.
• Validated methods enable better atmospheric modeling of Criegee intermediate chemistry.