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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.

A recently developed model chemistry (denoted as junChS [Alessandrini et al J. Chem. Theory Comput. 2020, 16, 988-1006]) has been extended to the employment of explicitly-correlated (F12) methods. This led us to propose a family of effective, reliable and parameter-free schemes for the computation of accurate interaction energies of molecular complexes ruled by non-covalent interactions. A thorough benchmark based on a wide range of interactions showed that the so-called junChS-F12 model, which employs cost-effective revDSD-PBEP86-D3(BJ) reference geometries, has an improved performance with respect to its conventional counterpart and outperforms well-known model chemistries. Without employing any empirical parameter and at an affordable computational cost, junChS-F12 reaches sub-chemical accuracy. Accurate characterizations of molecular complexes are usually limited to energetics. To take a step forward, the conventional and F12 composite schemes developed for interaction energies have been extended to structural determinations. A benchmark study demonstrated that the most effective option is to add MP2-F12 core-valence correlation corrections to fc-CCSD(T)-F12/jun-cc-pVTZ geometries without the need of recovering the basis set superposition error and the extrapolation to the complete basis set., Accepted on JCTC

Dimethyl sulfide (DMS) is the main organosulfur compound in the atmosphere. Oxidation by OH radicals to form the methyl thiomethyl species (MTMr) has been studied under normal atmospheric conditions and in reaction chambers at different O2 partial pressure, including complete absence of oxygen. Scarce attention has been devoted however to the possibility of further reaction of OH with MTMr. We present here a computational study using DFT, CCSD(T) and composite methods, on the properties of two stable intermediates never fully investigated before, methanesulfenyl methanol (MSMOH) and S-methyl-methanesulfenic acid (SMMSA), arising from addition of a second OH radical to MTMr.

Chemistry is a visual discipline deeply relying on graphical representations for bringing the structure and behaviour of the microscopic world at the reach of our senses. Recent advances in computer graphics and the advent of immersive-virtual-reality technologies are creating great new opportunities in the way of conceiving these graphical representations and interacting with them. To seize these opportunities we have developed a virtual laboratory casting researchers amid the electron clouds of molecular systems and allowing them to perform a quantitative analysis of chemical bonding in an interactive and cooperative way. We discuss the developed machinery and illustrate an example of bond analysis on group 11 cyanides M–CN (M = Cu, Ag, Au).Chemistry is a visual discipline deeply relying on graphical representations for bringing the structure and behaviour of the microscopic world at the reach of our senses. Recent advances in computer graphics and the advent of immersive-virtual-reality technologies are creating great new opportunities in the way of conceiving these graphical representations and interacting with them. To seize these opportunities we have developed a virtual laboratory casting researchers amid the electron clouds of molecular systems and allowing them to perform a quantitative analysis of chemical bonding in an interactive and cooperative way. We discuss the developed machinery and illustrate an example of bond analysis on group 11 cyanides M–CN (M = Cu, Ag, Au).