Sviluppo di metodi per la dinamica molecolare polarizzabile ed il calcolo di struttura elettronica

The computational description of chemical systems is nowadays acknowledged of great importance for the widespread application of theoretical methods in microscopic structural modelling. The modelling of both simple and complex molecular systems is feasible by applying the first principles of quantum mechanics and such simulations may be accomplished with or without their evolution in time. These approaches have their respective advantages and disadvantages, depending on the application field. In the present Thesis, two innovative methods for chemical modelling are presented, that is the Normal Mode Analysis within Extended Broken Symmetry (EBS) and the Fluctuating-RESP (F-RESP) method. The Extended Broken Symmetry (EBS) approach is an electronic structure method that permits to correct the Density Functional Theory (DFT) (or, more generally mono-determinant based methods) when dealing with low spin open-shell molecular systems characterized by antiferromagnetic interactions. This method (relying on ORCA or CP2K program packages for ab initio calculations) is generalized with respect to the system size, meaning that it can deal with molecular species containing an arbitrary number of spin centres. The analytical expressions for the geometrical gradient and Hessian matrix have been derived, therefore permitting to perform geometrical optimizations and provide vibrational spectra. The Fluctuating-RESP (F-RESP) approach is a framework for performing polarizable classical Molecular Dynamics simulations within the fluctuating charges formalism. The method (implemented in LAMMPS package) is able to perform, due to the remarkable parallelization and intrinsic theoretical feasibility, simulations within the limits of the most employed statistical ensembles. The former method has been applied in my study on two iron-sulfur clusters, representing the most common structural patterns of this important class of compounds and for which high quality experimental data are available. The accuracy of the description that EBS was able to perform of these systems superseded the traditional BS-DFT approach both at the magnetic, structural and vibrational levels. The latter method has been employed for the simulation of liquid water, a system of paramount importance whose knowledge is, despite of the apparent structural simplicity and huge research efforts made so far, still not thoroughly complete. The water model obtained by applying the F-RESP approach on the pre-existing TIP3P model permitted to improve the accuracy of the description and derive physical observables that are closer to the experimentally measured ones.