Yes / The operation of many nanostructured biomolecular sensors and catalysts critically hinges on the manipulation of non-covalent adsorption of biomolecules on unfunctionalised noble-metal nanoparticles (NMNPs). Molecular-level structural details of the aqueous biomolecule/NMNP interface are pivotal to the successful realisation of these technologies, but such experimental data are currently scarce and challenging to obtain. Molecular simulations can generate these details, but are limited by the assumption of non-preferential adsorption to NMNP features. Here, via first principles calculations using a vdW-DF functional, and based on nanoscale sized NMNPs, we demonstrate that adsorption preferences to NP features vary with adsorbate chemistry. These results show a clear distinction between hydrocarbons, that prefer adsorption to facets over edges/vertices, over heteroatomic molecules that favour adsorption onto vertices over facets. Our data indicate the inability of widely used force-fields to correctly capture the adsorption of biomolecules onto NMNP surfaces under aqueous conditions. Our findings introduce a rational basis for the development of new force-fields that will reliably capture these phenomena.

<p>Density functional theory is a formally exact theory to describe ground state properties due to the existence of the exact functional. In practice, the usefulness of density functional theory relies on the accuracy of density functional approximations. After decades of effort of functional developments, the present-day state-of-the-art density functional approximations have achieved reasonably good accuracy for small systems. However, the error grows with system size. One of the dominant errors intrinsic in the mainstream density functional approximations is the delocalization error, which arises because of the violation of Perdew-Parr-Levy-Balduz (PPLB) linearity condition. The PPLB condition governs the formulation of the density functional theory for fractional-charge systems, for which the ground state energy for the exact functional, as a function of the fractional electron number, should yield a series of line-segments across the integer points. In this dissertation, by imposing the PPLB condition in a local, size-consistent way, we develop the local scaling correction (LSC) and its updated version, the localized orbital scaling correction (LOSC), which largely improve upon the mainstream density functional approximations across system sizes. With the LOSC, we open a door towards a systematic elimination of delocalization error. Besides the ground state functional development, we also develop a gentlest ascent dynamics approach for accessing the excited states via time-independent ground state density functionals. This is also useful for exploring Kohn-Sham energy landscapes of approximate density functionals. I will also review the PPLB formulation of density functional theory for fractionally charged systems, and show that it is equivalent to the formulation normally used for fractional system calculations under certain assumptions. Furthermore, I will examine the behavior of the fractional system energy as a function of the fractional number of electrons for different mainstream functionals, and relate it to their errors for integer systems.</p> / Dissertation

Simulações de propriedades de estrutura eletrônica possuem fundamental importância para a física do estado sólido e química quântica. A teoria do funcional da densidade (DFT) é atualmente o método de estrutura eletrônica mais empregado, desde escalas atômicas e nanoscópicas até aglomerados biomoleculares. A acurácia da DFT depende essencialmente de aproximações para os efeitos de troca e correlação, para as quais existem vínculos a serem satisfeitos como forma de controlar sua construção. Esse é um tópico de grande importância, pois a construção de melhores funcionais é necessária para uma descrição cada vez mais precisa dos efeitos de muitos corpos na DFT. No presente trabalho, investigamos o comportamento da energia de troca e correlação e o desenvolvimento de funcionais aproximados sob a ótica de um vínculo universal de sistemas de interação Coulombiana, o limite inferior de Lieb-Oxford. Primeiramente apresentamos evidências de que em diversas classes de sistemas a energia de troca e correlação é distante do limite de Lieb-Oxford. A redução do limite foi implementada nos funcionais Perdew-Burke-Erzenhof (PBE), porém a forma com que o vínculo é implementado apenas aumentou a energia de troca. Propusemos em seguida que o limite de Lieb-Oxford não fosse utilizado apenas para determinar o valor de um parâmetro, como em PBE, mas que fosse ponto-de-partida de uma nova forma família de funcionais, do tipo hiper-GGA. Exploramos uma construção não-empírica, com implementação pós-autoconsistente. A particular forma proposta se beneficiou da redução do limite Lieb-Oxford, obtendo resultados satisfatórios para as energias de correlação. / Electronic-structure calculations play a fundamental role in solid-state physics and quantum chemistry. Density-functional theory (DFT) is today the most-widely used electronic-structure method, from atomic and nanoscopic scales to biomolecular aggregates. The accuracy of DFT depends essentially on approximations to the exchange and correlation energy, which are controlled by exact constraints. This is a very important issue, since the improvement of functionals is the key to a better description of many-body effects. In the present work, we investigate the exchange-correlation energy and approximate functionals from the viewpoint of an universal constraint on interacting Coulomb systems: the Lieb-Oxford lower bound. Initially we present evidence that for several classes of systems (atoms, ions, molecules and solids), the actual exchange-correlation energies are far from the Lieb-Oxford lower bound. A tighter form of this bound was conjectured; implemented in the Perdew-Burke-Erzenhof (PBE) functionals, and tested for atoms, molecules and solids. Finally, we propose to use the Lieb-Oxford bound not just to fix the value of a parameter as in PBE, but as a starting point for a new family of hyper-GGA functionals. For these, we explored a non-empirical construction, investigating its performance for atoms and small molecules post-selfconsistently. The particular HGGA proposed benefited from the tightening of the Lieb-Oxford bound and exhibited satisfactory correlation energies.

The chemistry of actinides in aqueous solution is important, and it is essential to build adequate conceptual models and develop methods applicable for actinide systems. The complex electronic structure makes benchmarking necessary. In the thesis a prototype reaction of the water exchange reaction for uranyl(VI), for both ground and luminescent states, described with a six-water model, was used to study the applicability of density functional methods on actinides and different solvation models. An excellent agreement between the wave function methods CCSD(T) and MP2 was obtained in the ground state, implying that near-minimal CASPT2 can be used with confidence for the reaction in the luminescent state of uranyl(VI), while density functionals are not suited to describe energetics for this type of reaction. There was an ambiguity concerning the position of the waters in the second hydration sphere. This issue was resolved by investigating a larger model, and prop- erly used the six-water model was found to adequately describe the water exchange reaction. The effect of solvation was investigated by comparing the results from conductor-like polarizable continuum models using two cavity models. Scattered numbers made it difficult to determine which solvation model to use. The final conclusion was that the water exchange reaction in the luminescent state of uranyl(VI) should be addressed with near-minimal CASPT2 and a solvation model without explicit cavities for hydrogens. Finally it was shown that no new chemistry appears in the luminescent state for this reaction. The thesis includes a methodological investigation of a multi-reference density functional method based on a range separation of the two-electron interaction. The method depends on a universal parameter, which has been determined for lighter elements. It is shown here that the same parameter could be used for actinides, a prerequisite for further development of the method. The results are in that sense promising.

L'interaction entre la matière et le rayonnement est un domaine bien établi de la physique. Pour un physico-chimiste, cette interaction peut être utilisée comme une sonde (spectroscopie) ou pour provoquer des réactions chimiques (photo-chimie). Les mécanismes des réactions photochimiques sont difficiles à étudier expérimentalement et même les études les plus sophistiquées de spectroscopies femtosecondes peuvent bénéficier énormément des simulations théoriques.Les résultats spectroscopiques d'ailleurs ont souvent besoin des calculs théoriques pour l'analyse de leurs spectres. Les méthodes théoriques pour décrire les processus photochimiques ont été principalement développées en utilisant le concept de la fonction d'onde à N corps et ont eu des succès remarquables. Cependant de telles approches sont généralement limitées à des petites ou moyennes molécules. Heureusement la théorie de la fonctionnelle de la densité dépendant du temps (TD-DFT) a émergé comme une méthode simple de calcul pouvant être appliquée à des molécules plus grandes, avec une précision qui est souvent, mais pas toujours, semblable à la précision provenant des méthodes basés sur la fonction d'onde à N électrons. Une partie de cette thèse consiste à surmonter les difficultés des approximations utilisées de nos jours en TD-DFT. En particulier, nous avons examiné la qualité des intersections coniques quand l'approche du retournement de spin non collinéaire de Ziegler-Wang est utilisée et nous avons montré que l'approche du retournement de spin, parfois ,améliore dans des cas particuliers, mais que c'est n'est pas une solution générale pour mieux décrire les intersections coniques dans les simulations photochimiques basées sur la TD-DFT. La plupart des parties de cette thèse traite d'améliorations algorithmiques, soit pour améliorer l'analyse des résultats de la TD-DFT, soit pour étendre les calculs de TD-DFT à de grandes molécules. L'implémentation de l'analyse automatique des symétries des orbitales moléculaires dans deMon2k est une contribution pour améliorer l'analyse des résultats de la TD-DFT. Cela a aussi servi comme une introduction au projet de programmation majeur. La contribution méthodologique principale dans cette thèse est l'implémentation des équations de Casida dans le code BigDFT fondé sur le formalisme des ondelettes. Cette implémentation a aussi permis une analyse détaillée des arguments positifs et négatifs de l'utilisation de la TD-DFT fondée sur les ondelettes. On montre qu'il est plus facile d'obtenir des orbitales moléculaires précises qu'avec deMon2k. Par contre, la contribution des orbitales inoccupées est plus problématiques qu'avec un code de gaussienne comme deMon2k. Finalement, les équations de base des gradients analytiques des états excités sont dérivées pour la TD-DFT. La thèse se termine avec quelques perspectives de travaux futurs. / The interaction of light with matter is a well-established domain of physical science. For a chemical physicist, this interaction may be used as a probe (spectroscopy) or to induce chemical reactions (photo- chemistry.) Photochemical reaction mechanisms are difficult to study experimentally and even the most sophisticated modern femtosecond spectroscopic studies can benefit enormously from the light of theoret- ical simulations. Spectroscopic assignments often also require theoreti- cal calculations. Theoretical methods for describing photoprocesses have been developed based upon wave-function theory and show remarkable success when going to sophisticated higher-order approxi- mations. However such approaches are typically limited to small or at best medium-sized molecules. Fortunately time-dependent density- functional theory (TD-DFT) has emerged as a computationally-simpler method which can be applied to larger molecules with an accuracy which is often, but not always, similar to high-quality wave-function calculations. Part of this thesis concerns overcoming difficulties in- volving the approximate functionals used in present-day TD-DFT. In particular, we have examined the quality of conical intersections when the Ziegler-Wang noncollinear spin-flip approach is used and have shown that the spin-flip approach has merit as a particular solution in particular cases but is not a general solution to improving the de- scription of conical intersections in photochemical simulations based upon TD-DFT. Most of this thesis concerns algorithmic improvements aimed at either improving the analysis of TD-DFT results or extending practical TD-DFT calculations to larger molecules. The implementa- tion of automatic molecular orbital symmetry analysis in deMon2k is one contribution to improving the analysis of TD-DFT results. It also served as an introduction to a major programming project. The major methodological contribution in this thesis is the implementation of Casida's equations in the wavelet-based code BigDFT and the subse- quent analysis of the pros and cons of wavelet-based TD-DFT where it is shown that accurate molecular orbitals are more easily obtained in BigDFT than with deMon2k but that handling the contribution of unoccupied orbitals in wavelet-based TD-DFT is potentially more problematic than it is in a gaussian-based TD-DFT code such as de- Mon2k. Finally the basic equations for TD-DFT excited state gradients are derived. The thesis concludes with some perspectives about future work.

Simulações de propriedades de estrutura eletrônica possuem fundamental importância para a física do estado sólido e química quântica. A teoria do funcional da densidade (DFT) é atualmente o método de estrutura eletrônica mais empregado, desde escalas atômicas e nanoscópicas até aglomerados biomoleculares. A acurácia da DFT depende essencialmente de aproximações para os efeitos de troca e correlação, para as quais existem vínculos a serem satisfeitos como forma de controlar sua construção. Esse é um tópico de grande importância, pois a construção de melhores funcionais é necessária para uma descrição cada vez mais precisa dos efeitos de muitos corpos na DFT. No presente trabalho, investigamos o comportamento da energia de troca e correlação e o desenvolvimento de funcionais aproximados sob a ótica de um vínculo universal de sistemas de interação Coulombiana, o limite inferior de Lieb-Oxford. Primeiramente apresentamos evidências de que em diversas classes de sistemas a energia de troca e correlação é distante do limite de Lieb-Oxford. A redução do limite foi implementada nos funcionais Perdew-Burke-Erzenhof (PBE), porém a forma com que o vínculo é implementado apenas aumentou a energia de troca. Propusemos em seguida que o limite de Lieb-Oxford não fosse utilizado apenas para determinar o valor de um parâmetro, como em PBE, mas que fosse ponto-de-partida de uma nova forma família de funcionais, do tipo hiper-GGA. Exploramos uma construção não-empírica, com implementação pós-autoconsistente. A particular forma proposta se beneficiou da redução do limite Lieb-Oxford, obtendo resultados satisfatórios para as energias de correlação. / Electronic-structure calculations play a fundamental role in solid-state physics and quantum chemistry. Density-functional theory (DFT) is today the most-widely used electronic-structure method, from atomic and nanoscopic scales to biomolecular aggregates. The accuracy of DFT depends essentially on approximations to the exchange and correlation energy, which are controlled by exact constraints. This is a very important issue, since the improvement of functionals is the key to a better description of many-body effects. In the present work, we investigate the exchange-correlation energy and approximate functionals from the viewpoint of an universal constraint on interacting Coulomb systems: the Lieb-Oxford lower bound. Initially we present evidence that for several classes of systems (atoms, ions, molecules and solids), the actual exchange-correlation energies are far from the Lieb-Oxford lower bound. A tighter form of this bound was conjectured; implemented in the Perdew-Burke-Erzenhof (PBE) functionals, and tested for atoms, molecules and solids. Finally, we propose to use the Lieb-Oxford bound not just to fix the value of a parameter as in PBE, but as a starting point for a new family of hyper-GGA functionals. For these, we explored a non-empirical construction, investigating its performance for atoms and small molecules post-selfconsistently. The particular HGGA proposed benefited from the tightening of the Lieb-Oxford bound and exhibited satisfactory correlation energies.

No description available.

Warm dense matter (WDM) is an extreme state of matter induced by extreme conditions and characterized as an intermediary state between (high-pressure) condensed matter and plasma. It has sparked a lot of attention in recent years as a result of current innovations in experiments and theoretical methods for modeling such complex systems. Such conditions naturally occur in astrophysical objects such as the interiors of the planets, and in white and brown dwarfs. WDM can be created in the laboratory via various methods such as laser compression, Z-pinches and heated diamond anvil cells. This thesis describes the results obtained for many such systems across a range of conditions modeled using ab-initio simulation methods. The first testbed concerns the electronic structure and linear response of the carbon phases under high-pressure and warm dense matter conditions. The focus is on modeling inelastic x-ray scattering spectra across a range of conditions useful for the analysis and interpretation of x-ray Thomson scattering (XRTS) experiments. Another major goal is to improve the existing models to compute static properties such as the equation of state, density of states with the inclusion of highly accurate data from quantum Monte Carlo (QMC) simulations relevant at finite-temperatures. This approach improves the accuracy and is also computationally inexpensive compared to path integral Monte Carlo (PIMC) methods. Lastly, improvements in linear response theory relevant for XRTS are incorporated with the inclusion of local field corrections (LFC) and finite-temperature local field corrections (T-LFC) using data from QMC simulations.

Cette thèse présente différentes approches quantiques pour l'exploration de processus dynamiques dans des systèmes multiélectroniques, en particulier après une forte excitation qui peut aboutir à des effets dissipatifs. Les théories de champ moyen sont un outil utile à cet égard. Malgré l'existence de nombreux travaux réalisés ces deux dernières décennies, ces théories peinent à reproduire complètement la corrélation à deux corps. La thermalisation est un des effets des collisions électron-électron. Après un chapitre introductif, on présentera dans le chapitre 2 le formalisme de plusieurs méthodes étudiées dans cette thèse, ayant pour but la description de ces effets en ajoutant un terme de collision au champ moyen. Ces méthodes sont appelées Stochastic Time-Dependent Hartree Fock (STDHF), Extended TDHF (ETDHF) et Collisional TDHF (CTDHF). Cette dernière méthode représente d'une certaine façon le résultat principal de cette thèse. L'implémentation numérique de chacune de ces méthodes sera aussi examinée en détail. Dans les chapitres 3, 4 et 5, nous appliquerons à différents systèmes les méthodes présentées dans le chapitre 2. Dans le chapitre 3, nous étudions d'abord un canal de réaction rare, ici la probabilité d'un électron de s'attacher à un petit agrégat d'eau. Un bon accord avec les données expérimentales a été observé. Dans le chapitre 4, un modèle fréquemment utilisé en physique nucléaire est résolu exactement et comparé quantitativement à STDHF. L'évolution temporelle des observables à un corps s'accorde entre les deux méthodes, plus particulièrement en ce qui concerne le comportement thermique. Néanmoins, pour permettre une bonne description de la dynamique, il est nécessaire d'avoir une grande statistique, ce qui peut être un frein à l'utilisation de STDHF sur de larges systèmes. Pour surpasser cette difficulté, dans le chapitre 5 nous testons CTDHF, qui a été introduit dans le chapitre 2, sur un modèle à une dimension (et sans émission électronique). Le modèle se compose d'électrons dans un potentiel de type jellium avec une interaction auto-cohérente sous la forme d'une fonctionnelle de la densité. L'avantage de ce modèle à une dimension est que les calculs STDHF sont possibles numériquement, ce qui permet une comparaison directe aux calculs CTDHF. Dans cette étude de validité du concept, CTDHF s'accorde remarquablement bien avec STDHF. Cela pose les jalons pour une description efficace de la dissipation dans des systèmes réalistes en trois dimensions par CTDHF. / This thesis presents various quantal approaches for the exploration of dynamical processes in multielectronic systems, especially after an intense excitation which can possibly lead to dissipative effects. Mean field theories constitute useful tools in that respect. Despite the existence of numerous works during the past two decades, they have strong difficulties to capture full 2-body correlations. Thermalization is one of these effects that stems from electron-electron collisions. After an introductory chapter, we present in Chapter 2 the formalism of the various schemes studied in this thesis toward the description of such an effect by including collisional terms on top of a mean field theory. These schemes are called Stochastic Time-Dependent Hartree Fock (STDHF), Extended TDHF (ETDHF) and Collisional TDHF (CTDHF). The latter scheme constitutes in some sense the main achievement of this thesis. The numerical realizations of each scheme are also discussed in detail. In Chapters 3, 4 and 5, we apply the approaches discussed in Chapter 2 but in various systems. In Chapter 3, we first explore a rare reaction channel, that is the probability of an electron to attach on small water clusters. Good agreement with experimental data is achieved. In Chapter 4, a model widely used in nuclear physics is exactly solved and quantitatively compared to STDHF. The time evolution of 1-body observables agrees well in both schemes, especially what concerns thermal behavior. However, to allow a good description of the dynamics, one is bound to use a large statistics, which can constitute a hindrance of the use of STDHF in larger systems. To overcome this problem, in Chapter 5, we go for a testing of CTDHF developed in Chapter 2 in a one-dimensional system (and without electronic emission). This system consists in electrons in a jellium potential with a simplified self-consistent interaction expressed as a functional of the density. The advantage of this 1D model is that STDHF calculations are numerically manageable and therefore allows a direct comparison with CTDHF calculations. In this proof of concept study, CTDHF compares remarkably well with STDHF. This thus paves the road toward an efficient description of dissipation in realistic 3D systems by CTDHF.

This thesis presents a theoretical study of materials under high pressure using ab initio lattice dynamics based on density functional theory and density functional perturbation theory using both super-cell and linear response approach. Ab initio lattice dynamics using super-cell approach is applied to compare our theoretical predictions with experimental findings. Phonon dispersion curves of fcc α-γ cerium are calculated and compared with inelastic X-ray scattering data. Pressure dependency of phonon density of states in two cubic phases TiO2 allows us to assign the observed cubic phase in experiments to be of fluorite rather than pyrite structure. Dynamical stability of cotunnite TiO2 phase at low pressure can explain the observed quenching phenomena in experiments. Our calculated O2 vibron mode in both ε-ζ phases of solid oxygen supports the hypothesis that both phases are iso-structural. Hydrogen-dense materials attract great attention not only because they open a path to study phenomena related to metallization (superconductivity) of solid hydrogen but also because they are closely related to important industrial applications (hydrogen storage). Using linear response method, we find that metallic fcc-AlH3 is dynamically stabilized in the range of 72-106 GPa and can persist at ambient pressure if finite temperature effects are considered. For SiH4, we test dynamical stability, Raman spectra, zero point energy, and utilize GW calculations for self energy correction. We find that a metallic tetragonal phase of SiH4 can be assigned to the experimentally observed one. Our ab initio lattice dynamics calculations based on density functional perturbation theory predict that fcc-YH3 is a pressure-induced superconductor with a high transition temperature of 40 K at 17.7 GPa. With increasing pressure this material undergoes a superconductor-metal-superconductor transition and the underlying mechanism of this transition can simultaneously explains also the observed metal-insulator transition at 25 GPa in YH3-δ.