Advanced computational modelling of structural and dynamic properties of complex systems.

This doctoral dissertation focuses on the theoretical and computational modeling of the electronic structure of complex systems. The study of chemical systems with several degrees of freedom requires the proper selection of contributions in order to simplify the system and develop a physical-chemical model. In this context, the characterization of the electronic structure is fundamental. Depending on the experimental framework, two approaches were applied: ab initio calculations and simulations of ab initio Molecular Dynamics (AIMD). On the basis of these premises, a variety of experimental systems have been investigated. The Brandi-Guarna reaction has been investigated in order to interpret the lack of regioselectivity in the cycloaddition reaction that produces the 5-isoxazolidine compound and the syntesis of the piridone and enaminone side product by the thermal rearrangement of the isoxazolidine. The theoretical investigation of the tautomerization of [2,2’-bipyridyl]-3,3’-diol allowed us to evaluate the accuracy of the exchange-correlation functional at a chosen level of theory. The tautomerization involves a double proton and is substantially correlated with both the energy of the first transition state and the delocalization error of the adopted functional. The computational results show that the step-by-step mechanism of tautomerism can only be bound in xc-functionals with high levels of both terms. The computational study, done in collaboration with an experimental research group, has produced a useful tool for the interpretation of the experimental re-sults. The ab initio optimization of Posner-like clusters show that the addition of Mg2+ to Amorphous Calcium Phosphate samples stabilizes the amorphous phase against heating. The simplified computational model contributes to the formulation of guidelines for simple synthetic approaches to synthetize ACP nanoparticles with a customized elemental composition. The ab initio modeling of the adenine adsorption process on a gold surface (111) shows that the two selected tautomeric forms (N9 and N7) have substantially distinct behaviors. The steric hindrance given by the different protonated nitrogens is a decisive factor in the adsorption process. The presence of a tautomerization process between the two tautomeric forms of adenine would explain the sometimes contradicting conclusions found in the scientific literature. These results suggest further investigations into the mechanism of water-mediated tautomerization of adenine when adsorbed on the metal surface.