Exploring the Theoretical Kinetics Frontiers: Charting the Path from Methodology to Application in Barrierless Reaction Characterization

This thesis presents the development and application of advanced computational protocols for the accurate characterization of gas-phase reactions of interest in astrochemistry and interstellar medium (ISM) chemistry.The research focuses on several key areas: Characterization of stationary points: A new quantum chemical model, the Pisa Composite Scheme (PCS), has been developed to describe stationary points and reaction energetics accurately. This black-box procedure remarkably balances accuracy and computational cost for medium-sized molecules, including closed-shell and open-shell systems. The PCS provides highly accurate energies and thermochemical properties, which are crucial for understanding reaction mechanisms and kinetics.Geometry refinement, zero-point energies, and partition functions: The BPCS (Bond-corrected Pisa Composite Scheme) has been proposed for geometry optimization. This approach combines the accuracy of a double-hybrid functional with a triple-zeta basis set for structure determination and relative energies, while employing a more economical hybrid functional with a double-zeta basis set for zero-point energies and vibrational partition functions within the VPT2 framework.Characterization of reactions:              - For barrierless reactions: A new protocol to derive in a black-box way the most suitable exchange-correlation functional has been derived and implemented in a develoment version of EStokTP software. Then, the thesis introduces an enhanced protocol combining PCS, I-DDCI (Iterative Difference-Dedicated Configuration Interaction), and MC-PDFT (Multi-Configuration Pair-Density Functional Theory) methods. This innovative approach ensures accurate computation of reactive fluxes in barrierless entrance channels, crucial for fields such as combustion processes, atmospheric chemistry, and astrochemistry.              - For reactions with barriers: An innovative approach has been developed to handle the transition from harmonic vibrations to hindered rotations along the intrinsic reaction coordinate (IRC). This method successfully treats the transition between the two limiting behaviours in a smooth way, providing a more accurate description of the reaction dynamics, also when the traditional harmonic approximation breaks down.Reduced-cost approach for dissociation curves: A novel method for obtaining accurate dissociation curves at reduced computational cost has been proposed and validated for a large panel of prototypical reactions. The unsupervised protocol selects the most accurate functional form of the dissociation potential having the correct asymptotic limits with a maximum of 5 points computed at pre-determined inter-fragment distances.The developed methods and protocols represent, in the author opinion, significant advancements in computational chemistry, providing robust tools employable also by non-specialists for characterizing reaction pathways and molecular properties with high accuracy and efficiency. These features should improve the synergism between theory and experiments in the study of the complex chemistry of extreme environments like the ISM, where traditional experimental methods face significant challenges.Future perspectives include the further refinement of the I-DDCI method thanks to a more flexible partitioning of the orbital space. The envisaged enhancements rooted in the definition of frozen occupied orbitals, perturbing occupied orbitals, and several classes of virtual orbitals, aim to extend the applicability of these methods to larger molecular systems without any accuracy reduction.In the author opinion, this work contributes to bridging the gap between computational efficiency and experimental precision in the study of gas-phase reactions and interstellar chemistry. By providing a comprehensive suite of computational tools, it enables more accurate modeling of complex chemical processes under extreme conditions, advancing our understanding of molecular formation and evolution in diverse environments, from combustion systems to the interstellar medium.