Este projeto tem dois objetivos. Primeiramente estudou-se a aplicabilidade dos splines tricúbicos para a construção de superfícies de energia potencial globais. Um dos obstáculos que este método tem de superar e a escolha de um sistema de coordenadas apropriado, que minimize a influência de pontos não físicos. Para isto, propôs-se o uso do sistema de coordenadas de Pekeris, nunca usado para este fim. Este procedimento foi realizado para três sistemas químicos bem descritos na literatura, [Cl,H2], [F,H,D] e [H,O,Cl], cujas superfícies de energia potencial e propriedades das reações foram usadas como referência. Com base nestes modelos, aplicamos o método proposto variando-se a quantidade e a disposição dos nós das interpolações, a fim de verificar sua influência na qualidade das superfícies interpoladas. Os resultados mostram que as superfícies construídas por este método reproduzem muito bem os cálculos de dinâmica química, tanto por métodos quânticos quanto por métodos clássicos. Para isto, os nós da interpolação devem cobrir as regiões mais importantes da superfície de energia potencial e os valores mais baixos das coordenadas de Pekeris devem ser priorizados. O segundo objetivo consiste na aplicação deste procedimento na construção da superfície de energia potencial [H,S,F]. Com esta superfície, diversas características deste sistema foram analisadas, tais como geometrias dos pontos estacionários, energias relativas e frequências vibracionais. Os valores obtidos estão de acordo com os dados descritos na literatura. A superfície construída também foi usada para a realização de cálculos de dinâmica para a reação F+HS → S+FH. Observamos a existência de dois tipos de mecanismos, um com a formação de um intermediário de longa duração e outro com a abstração direta do átomo de hidrogênio. / This project has two goals. First, we studied the applicability of the tricubic splines to construct global potential energy surfaces. One of the diculties this approach has to overcome is the choice of an appropriate coordinate system that minimises the in uence of non-physical points. For such, we proposed the use of the Pekeris coordinate system, never employed for this purpose. This procedure was carried out for three well described systems, [Cl,H2], [F,H,D] and [H,O,Cl], whose potential energy surfaces and reaction properties were taken as references. Based on these models, we applied the proposed method varying the amount and arrangement of the interpolation knots, to verify their influence on the quality of the interpolated surfaces. The results showed that surfaces constructed by this approach reproduce very well the chemical dynamics calculations, both for the quantum as well as for the classical methods, provided that the interpolation knots cover the most important regions of the potential energy surfaces, and the lower values of the Pekeris coordinates are prioritised. The second goal was the application of this procedure to the construction of the [H,S,F] potential energy surface. With this surface, several characteristics of this system were analysed, such as the geometry of the stationary points, relative energies and vibrational frequencies. The values obtained are in agreement with the data described in the literature. The constructed surface was also used for quantum dynamics calculations on the reaction F + HS → S + FH. We observed two kinds of mechanisms, one of them with the formation of a long-living intermediate and the other with the direct abstraction of the hydrogen atom.

No description available.

Une littérature scientifique nourrie est consacrée aux processus élémentaires hétérogènes se produisant à l’interface gaz-solide en raison de leur rôle clé dans de nombreux domaines. Ainsi, l’interaction d’atomes et de molécules avec les surfaces revêt une importance primordiale dans l’étude de la catalyse hétérogène, la combustion, la corrosion, le stockage de l’hydrogène, l’industrie automobile et pétrolière, les interactions plasma/paroi dans le contexte du réacteur expérimental thermonucléaire (ITER), les technologies du spatial, la chimie atmosphérique et l’astrochimie, pour citer quelques exemples. Lorsqu'un atome ou une molécule entre en collision avec une surface, de nombreux processus élémentaires peuvent avoir lieu. Ils dépendent de nombreux facteurs tels que : l'énergie de collision du projectile, l'angle d'incidence sur la surface, la température de surface, l'état initial le des molécules, le transfert d'énergie entre la surface des projectiles, etc… Tous ces facteurs influencent fortement les mécanismes réactionnels et la dynamique de ces processus. Les expériences de faisceaux moléculaires permettent un contrôle toujours plus précis de l'état initial des réactifs associé à un caractérisation fine des produits de réactions. Cependant, dans la plupart des cas, ces observations expérimentales ne fournissent pas tous les détails qui nous permettent de décrire finement les mécanismes gouvernant les processus élémentaires étudiés. Par conséquent, l'élaboration de modèles théoriques devient essentielle pour en rationaliser la description. L'objectif principal de ce travail de thèse est de proposer une analyse de la dynamique de plusieurs processus élémentaires pouvant se produire sur une surface de W(100) en contact avec de l'hydrogène et de l'azote (diffusion inélastique de N2 et H2, l'adsorption dissociative et non dissociative de H2 et l'adsorption et l'absorption de H et N). Par rapport aux études antérieures, la nouveauté de ce travail réside dans la prise en compte des interactions à longue distance de type van der Waals, qui s’avèrent essentielles lorsqu'on souhaite atteindre un bon accord théorie expérience dans le régime des faibles énergies de collision. Les résultats sont comparés aux données expérimentales disponibles et aux résultats théoriques antérieurs. Des simulations de dynamique moléculaire quasi-classique sont réalisés à l'aide de surfaces d'énergie potentielle (PES) basées sur la théorie de la fonctionnelle de la densité, tenant compte d’interactions non locales, telles que les forces de van der Waals. La dissipation de l’énergie aux vibrations du réseau et aux excitations électroniques est prise en compte au moyen de modèles effectifs. / An important part of scientific literature is devoted to the heterogeneous elementary processes occurring at gas-solid interface due to their great importance and key role in many different domains and applications. Thus, interaction of gas atoms/molecules with surface reactions are of primary importance in the study of: heterogeneous catalysis, combustion of solid fuel and coal gasification, processes of corrosion, hydrogen storage in solid material, automotive and oil industry, plasma-wall interactions in the context of thermonuclear experimental reactor (ITER), atmospheric re-entries technologies and astrochemistry, to name some examples. When an atom or molecule impinges on a surface many different elementary processes can take place, which depends on factors such as: the collision energy of the projectile, the angle of incidence to the surface, the surface temperature, the initial state of the molecules, the transference of energy projectiles-surface, etc. All these factors determines the mechanisms of reaction and the dynamics of the processes. Experimental molecular beams (MB) and other experimental techniques are able to accurately control the initial state of the reactive and characterizing products of gas-surface reactions. However, in most of the case experimental techniques do not provide enough details about the mechanisms through which elementary processes occur. Consequently, theoretical models becomes essential to rationalize the description that in certain cases the experiments do not reach.The main goal of this thesis work is to propose an analyze of the dynamics of several elementary processes occurring on a W(100) surface, such as: the inelastic scattering of N2 and H2, the dissociative and non-dissociative adsorption of of H2 and the adsorption and absorption of H and N. Compared to previous studies, the novelty of this work resides in the taking into account of van der Waals long-distance interactions, which are essential to reach a good agreement between theoretical and experiment results, especially at low collision energy regime. To rationalize the non-adiabatic effects, the energy dissipation to lattice vibrations and electronic excitation are taken in to account by means of GLO and LDFA models respectively.

No description available.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal. / Des creux d’interférence sont fréquemment observés dans les spectres d’absorption de complexes d’éléments de transition. Ces creux résultent de l’interaction entre des états électroniques excités de multiplicité de spin différente. Un modèle de puits de potentiel couplés pour les états électroniques est nécessaire pour l’analyse des spectres d’absorption. Une équation analytique, dérivée de ce modèle, permet de calculer les spectres d’absorption non-résolus. L’impact d’une variation de chaque paramètre sur l’allure des spectres calculés est présenté. Le calcul avec l’équation analytique de spectres d’absorption peu résolus de complexes du nickel(II) de coordination octaédrique reproduit bien la région du maximum de la bande et du creux. Les paramètres obtenus sont quantitatifs et ont une signification physique. Le modèle est aussi employé pour l’analyse des spectres avec des progressions vibroniques résolues, mais dont les différences entre progressions sont non-constantes. Les calculs numériques exacts reproduisent bien les écarts entre les maxima des progressions et leur intensité. / Interference dips are often observed in absorption spectra of transition metal complexes. These dips resuit from the interaction between excited states with different spin multiplicities. A mode! of coupled potential wells for the electronic states is required to analyse the absorption spectra. The pararneters of an analytical equation derived from this modet are varied individually. The analytical equation is used to calculate the unresolved absorption spectra of octahedral nickel(II) complexes. The calculated spectra reproduce well the experimental ones, especially in the regions of the dip and of the maximum of the band. The values for the parameters ohtained from the equation are physically relevant, quantitative and give precise information on excited-state properties for the complexes studied. Numerical calculations of absorption spectra showing resolved progressions with nonconstant energy intervals are done with the model and reproduce well the intervals between the maxima of the progression and their intensities.

No description available.

Potential energy surfaces are fundamental to materials simulation, providing acomprehensive landscape of the energy variations as a function of atomic positions within a system. These surfaces are crucial for understanding the behavior of materials at the atomic and molecular levels. By mapping out the potential energy surface, researchers can predict stable configurations, transition states, and thermodynamic and kinetic observables, essential for the design of novel materials. However, constructing accurate potential energy surfaces presents significant challenges due to the complexity constructing a potential with nearly the same accuracy of quantum mechanical calculations, the transferability to explore the high-dimensional space of atomic configurations, and the efficiency to be evaluated without the need for extensive computational resources. In this work, group theoretical irreducible derivativemethods are used to construct accurate Taylor series expansions of the potential energy surface. A new condition number optimized basis is developed which guarantees no amplification of error at the smallest computational cost allowed by group theory. Using this highly efficient method, the physics of the metal-insulator transition in LuNiO₃ is investigated within both a purely harmonic model and simplified anharmonic model, yielding reasonable and good agreement with the experimental phase transition temperature, respectively. Then the irreducible derivatives approach is used to benchmark the phonon anharmonicity of the several popular machine learning potentials: Gaussian approximation potentials, artificial neural network potentials, and graph neural network potentials. The benchmark indicated that the graph neural network potentials had the ability to accurate reproduce quantum mechanical derivatives up to 5th-order. Finally, insights from the benchmark are used to train an accurate graph neural network potential for strongly correlated UO₂ which yielded excellent agreement with quantum mechanical calculations and experiment. These methodological advancements underscore the potential of combining grouptheoretical approaches with cutting-edge machine learning techniques to enhance the precision and efficiency of materials simulations. By achieving high accuracy in modeling complex phenomena such as phase transitions and phonon anharmonicity, this work paves the way for future studies to not only explore the design of novel materials with unprecedented properties, but to design new machine learning techniques where group theory is built from the ground up.

In this thesis a reduced dimensionality quantum scattering model is applied to the study of polyatomic reactions of type X + CH4 <--> XH + CH3. Two dimensional quantum scattering of the symmetric hydrogen exchange reaction CH3+CH4 <--> CH4+CH3 is performed on an 18-parameter double-Morse analytical function derived from ab initio calculations at the CCSD(T)/cc-pVTZ//MP2/cc-pVTZ level of theory. Spectator mode motion is approximately treated via inclusion of curvilinear or rectilinear projected zero-point energies in the potential surface. The close-coupled equations are solved using R-matrix propagation. The state-to-state probabilities and integral and differential cross sections show the reaction to be primarily vibrationally adiabatic and backwards scattered. Quantum properties such as heavy-light-heavy oscillating reactivity and resonance features significantly influence the reaction dynamics. Deuterium substitution at the primary site is the dominant kinetic isotope effect. Thermal rate constants are in excellent agreement with experiment. The method is also applied to the study of electronically nonadiabatic transitions in the CH3 + HCl <--> CH4 + Cl(2PJ) reaction. Electrovibrational basis sets are used to construct the close-coupled equations, which are solved via Rmatrix propagation using a system of three potential energy surfaces coupled by spin-orbit interaction. Ground and excited electronic surfaces are developed using a 29-parameter double-Morse function with ab initio data at the CCSD(T)/ccpV( Q+d)Z-dk//MP2/cc-pV(T+d)Z-dk level of theory, and with basis set extrapolated data, both corrected via curvilinear projected spectator zero-point energies. Coupling surfaces are developed by fitting MCSCF/cc-pV(T+d)Z-dk ab initio spin orbit constants to 8-parameter functions. Scattering calculations are performed for the ground adiabatic and coupled surface models, and reaction probabilities, thermal rate constants and integral and differential cross sections are presented. Thermal rate constants on the basis set extrapolated surface are in excellent agreement with experiment. Characterisation of electronically nonadiabatic nonreactive and reactive transitions indicate the close correlation between vibrational excitation and nonadiabatic transition. A model for comparing the nonadiabatic cross section branching ratio to experiment is discussed.

Les processus élémentaires hétérogènes à l’interface gaz-solide présentent un intérêt fondamental dans de nombreux domaines tels que la catalyse hétérogène, la chimie atmosphérique et des milieux interstellaires, la rentrée atmosphérique de véhicules spatiaux ou encore la description des interactions plama-paroi. Cette thèse a pour objet l’étude de la dynamique des processus de collision non réactive de l’azote N2 sur une surface de tungstène W(100) et des processus de recombinaison moléculaire de l’hydrogène H2 sur des surfaces de tungstène W(100) et W(110). Leur dynamique quasi classique est simulée au moyen de surfaces d’énergie potentielle préalablement construites à partir de calculs de théorie de la fonctionnelle de la densité. Un potentiel multi-adsorbats est notamment développé pour tenir compte du taux de couverture de surface afin d’étudier la compétition entre la recombinaison directe, de type Eley-Rideal et la recombinaison par « atomes chauds » après diffusion hyperthermique d’un atome sur la surface. / Heterogeneous elementary processes at the gas-solid interface are ofgreat interest in many domains such as heterogeneous catalysis, atmospheric and interstellar media chemistry, spacecraft atmospheric re-entry and plasma-wall interactions description. This thesis focus on the dynamics of nitrogen, N2, non reactive scattering on a tungsten W(100) surface and hydrogen, H2, recombination processes on tungsten surfaces W(100) and W(110). The quasiclassical dynamics of these processes is simulated using potential energy surfaces based on density functional theory calculations. In particular, a multi-adsorbate potential is developed to include surface coverage in the dynamics simulation in order to scrutinize the interplay between both direct abstraction, the so-called Eley-Rideal recombination,and the Hot-Atom recombination process after hyperthermal diffusion on the surface

Les travaux présentés dans cette thèse peuvent être divisés en deux parties. Dans une première partie, nous avons étudié le processus de photoisomérisation dans plusieurs systèmes. Une analyse de structure électronique accompagnée d’un calcul préliminaire de dynamique semi-classique ont été appliqué à un modèle minimal du rétinal afin d’extraire les degrés de libertés les plus importants lors de l’isomérisation. Cela dans le but de construire des surfaces d’énergie potentielle diabatiques pour effectuer une étude de dynamique quantique. Une approche de type dynamique semi-classique a été appliquée à un modèle de moteur moléculaire dans le but d’étudier l’origine de l’uni-directionalité de sa rotation. Finalement, une étude de structure électronique d’un complexe de Rhénium contenant un ligand de type rétinal a été effectué pour étudier l’influence du métal sur la spectroscopie du ligand rétinal. Dans une deuxième partie nous nous sommes intéressés à l’étude des croisements intersystème dans un complexe de Rhénium. Afin de pouvoir apporter une explication à un comportement contrintuitif de ce complexe, nous avons développé un Hamiltonien modèle capable de tenir compte des couplages vibroniques interétats et spin-orbit. Cet Hamiltonien a été testé sur ce-dit système, et nous a permis, grâce à une étude de structure électronique de proposer un mécanisme de relaxation différent de celui proposé expérimentalement. / This thesis can be divided in two parts.In the first one, we have studied the photoisomerization process in several systems. An electronic structure analysis mixed with a preliminary semi-classical dynamics investigation has been applied to a minimal model of the retinal chromophore in order to select the most important degrees of freedom involved in the process. The goal of this is to build diabatic potential energy surfaces in order to conduct quantum dynamics simulations. A semi-classical approach has also been applied to a molecular motor model to study the origin of the unidirectionality of its rotary motion. Finally, an electronic structure of a rhenium complex with a retinal-like ligand has been performed to study the effect of the coordination to a metallic atom on the spectroscopy of the retinal ligand. In the second part, we have investigated the intersystem crossings in a rhenium complex. In order to bring an explanation to an experimentally observed conterintuitive behavior of this complex, we have developed a model Hamiltonian that includes both interstate vibronic coupling and spin-orbit coupling. This Hamiltonian has been tested on the said complex and, in complement to an electronic structure study, allowed us to formulate a decay mechanism different from the one proposed based on experiments.