The development of quantum mechanics has historically allowed researchers to theoretically explore the fundamental physical properties of atoms and molecules. Although quantum mechanics has been around for almost a century, its use was largely limited by the computational complexity it demanded. In the past decade, computer technology has evolved to the point where it is possible to perform calculations on biologically relevant systems. This has allowed us to corroborate results obtained from experiment as well as predict and explain phenomena that experiment cannot. Unfortunately, the field as a whole has not progressed to the point where high level methods, such as Multi-Reference Configuration Interaction (MRCI), are applicable to large molecular systems. Thus, to effectively study these systems, compromises must be made. In this work, two different approaches are taken to study the photophysical properties of systems such as DNA. In the first approach, a model system is formulated and studied in lieu of the larger target system. The excited state dynamics of 8-oxoguanine (8-oG) and its anion are studied in order to assess the possibility of taking part in an electron transfer mechanism to repair a nearby cyclobutane pyrimidine dimer (CPD). It is found that barriers on the anion S1 excited state surface prohibits easy access to conical intersections with the ground state, causing the anion to have a much longer excited state lifetime than the neutral form. Although much insight can be gained by this method, it is not uncommon for crucial interactions to be lost through simplification. In this case, when 8-oG is placed in an adenine dinucleotide, the π stacking interaction allows it to form a long lived radical base pair, which may be fundamental to its role in CPD repair. Unfortunately, it is impossible to carry out the same excited state calculations for the 8-oG/adenine dinucleotide due to computational cost. For reasons such as these, we also implement and benchmark a new approach to carrying out high level configuration interaction calculations in which the MRCI is expanded in the basis of high multiplicity natural orbitals (HMNOs). Specifically, the HMNO approach is implemented by expanding the MRCI wavefunction in the basis of natural orbitals generated from a ground state high multiplicity Configuration Interaction Singles and Doubles (CISD) calculation. Excited state calculations both at and away from the Franck-Condon region were performed to benchmark the ability of the HMNO approach using CISD and MRCI to reproduce standard MRCI energies. The ability of the HMNOs to be truncated was also explored, yielding efficient truncation criteria and guidelines for choosing the best basis set. It is found that the MRCI/HMNO approach yields energies that are in excellent agreement with standard MRCI while only requiring a fraction of the computational effort, possibly allowing it to be applied to larger systems such as nucleotide dimers. / Chemistry

We are interested in complex electronic structures of various atomic and ionics systems. We use an ab initioapproach, the multiconfigurational Dirac-Hartree-Fock (MCDHF), to compute atomic structures and properties.We contribute in three main ways to the already existent literature: by developing and implementing originalcomputer programs, by investigating possibilities of alternative computational methodologies and strategies, andfinally by performing accurate atomic structure calculations to support other research fields, i.e. nuclear physics,astrophysics or experimental physics, through the theoretical estimation of relevant atomic data.We raise questions about the choice of the optimal orbital basis by considering finite basis sets, MCDHF orbitalbases and natural-orbital bases. We demonstrate the promising potential of the latter in the context of hyperfinestructures and hope that others will find interest in pursuing our analysis. Ultimately, our work put forward someweaknesses of the traditional optimization strategy based on the layer-by-layer optimization strategy.We also perform large-scale calculations to determine accurate atomic properties such as energy levels, hyperfinestructures, isotope shifts, transition parameters, radiative lifetimes and Landé g factors. We show through thevariety of atomic properties and atomic systems studied, the difficulty of describing, in the relativistic framework,the correlation between the spatial position of electrons due to their Coulomb repulsion.This thesis is organized in two main parts. The first one is dedicated to the theoretical and computationalbackgrounds that are needed to understand the theoretical models and the interpretation of our results. Thesecond part presents and summarizes our articles and manuscripts. They are separated in four groups, A, B, C,and D, around the themes of the atomic orbital bases, the applications to nuclear physics, the applications toastrophysics, and investigations of negative ions. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

The central problem in the modern electronic structure theory is the calculation of cor- relation energy, possibly by an approach that would account for both static and dynamic correlation in an efficient, balanced and accurate way. In this thesis, I present a collection of methods that combine the effective treatment of dynamic correlation by the coupled cluster theory with density matrix renormalization group, a well-established technique for calculations of strongly correlated systems. The connection between them is achieved via the tailored coupled clusters (TCC) ansatz, which conveniently does not impose any ad- ditional computational costs. After the successful initial assessment, we developed more efficient implementations of these methods by employing the local approaches based on pair natural orbitals. This way, we extended the range of possible applications to larger systems with thousands of basis functions. To assess the accuracy of TCC as well as its local counterparts, we performed a variety of benchmark calculations ranging from small, yet challenging systems such as the nitrogen molecule or tetramethyleneethane diradical, to larger molecules like oxo-Mn(Salen) or Fe(II)-porphyrin model. 1

In this thesis, several development steps for the incremental method are presented. At first, the extension of the incremental scheme to other quantities than the energy is advanced in terms of molecular dipole moments. In this context, a revised error correction as well as the template localization for the treatment of aromatic systems are introduced. As a second enhancement, a new implementation of the template localization ensures a higher stability of this algorithm step and, thus, of the incremental scheme. Finally, pair natural orbitals (PNOs) are utilized in the incremental method with the aim of an increased efficiency. The PNO approach is re-assessed in context of the incremental expansion leading to both small incremental and PNO local errors for reaction, intermolecular interaction and cluster binding energies. The higher efficiency due to the twofold reduction of the computational efforts by the PNO and the incremental approaches is demonstrated for molecular clusters. Additionally, the complete basis set (CBS) limit is targetted by using the efficient MP2-based focal-point approach to the incremental scheme (with and without PNOs). Finally, based on these improvements of the performance, the PNO-based incremental scheme is applied to support a computational study regarding the modelling of the reaction mechanism for the base-catalyzed twin polymerization.