Computational strategies for the accurate thermochemistry and kinetics of gas-phase reactions

This PhD thesis focuses on the theoretical and computational modeling of gas phase chemical reactions, with a particular emphasis on astrophysical and atmospherical ones. The ability to accurately determine the rate coefficients of key elementary reactions is deeply connected to the accurate determination of geometrical parameters, vibrational frequencies and, even more importantly, electronic energies and zeropoint energy corrections of reactants, transition states, intermediates and products involved in the chemical reaction, together with a suitable choice of the statistical approach for the rate computation (i.e. the proper transition state theory model). The main factor limiting the accuracy of this process is the computational time requested to reach meaningful results (i.e. reaching subchemical accuracy below 1 kJ mol−1), which increases dramatically with the the size of the system under investigation. For small-sized systems, several nonempirical procedures have been developed and presented in the literature. However, for larger systems the well-known model chemistries are far from being parameter-free since they include some empirical parameters and employ geometries which are not fully reliable for transition states and noncovalent complexes possibly ruling the entrance channels. Based on these premises, this dissertation has been focused on the development of new “cheap” composite schemes, entirely based on the frozen core coupled cluster ansatz including single, double, and (perturbative) triple excitation calculations in conjunction with a triple-zeta quality basis set, including the contributions due to the extrapolation to the complete basis set limit and core-valence effects using second-order Møller- Plesset perturbation theory. For the first time the “cheap” scheme has been extended to explicitly-correlated methods, which have an improved performance with respect to their conventional counterparts. Benchmarks with different sets of state of the art energy barriers, interaction energies and geometrical parameters spanning a wide range of values show that, in the absence of strong multireference contributions, the proposed models outperforms the most well-known model chemistries, reaching a subchemical accuracy without any empirical parameter and with affordable computer times. Besides the composite schemes development efforts, a robust protocol for disclosing the thermochemistry and kinetics of reactions of atmospheric and astrophysical interest, rooted in the so-called ab initio-transition-state-theory-based master equation approach have been thoroughly investigated and validated.