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The Tully’s fewest switches surface hopping algorithm is implemented within the framework of the time-dependent density functional based tight binding method (TD-DFTB) to simulate the energy relaxation following absorption of a UV photon by polycyclic aromatic hydrocarbons (PAHs). This approach is used to study the size effect on the ultrafast dynamics in excited states for a special class of PAH species called polyacenes. We determine the dynamical relaxation times and discuss the underlying mechanisms. Our results show that there is a striking alternation in decay times of the brightest singlet state for neutral polyacenes with 3 to 6 aromatic cycles. The alternation corresponds to an order-of-magnitude variation between roughly 10 and 100 fs and is correlated with a qualitatively similar alternation of energy gaps between the brightest state and the state lying just below in energy.

Collisions of electronically excited krypton dimers with krypton atoms are studied using a hybrid (quantum-classical) dynamical method, semi-empirical diatomics-in-molecules electronic Hamiltonian, and Monte Carlo modeling. Krypton dimer mobility in krypton gas and dimer disappearance rate constants have been calculated for a broad range of the reduced electric field and five lowest excited electronic states of the dimer ion. Comparison with calculations recently reported for the electronic ground-state krypton dimer ion and with available experimental data is also provided. Two groups of the electronic states of the krypton dimer ion, resulting from a spin-orbit induced splitting, have been analyzed separately. Importantly, for both groups of states, the theoretical results bracket the experimental ones, therefore, considering mixtures of electronically excited states may strongly improve the agreement between theory and experiment. In addition, the effect of rotational-vibrational excitations in electronically excited krypton dimer ions is assessed and shown to also lead to an improved agreement between theory and experiment.

Reaction rate constants have been calculated for electronic transitions in Kr + 2 ions and for their decay as induced by collisions with krypton atoms and/or spontaneous radiation processes. The rate constants have then been used in a series of modelings of electronic relaxation in the ions in cold krypton plasmas with the main focus on relaxation times and final states. It has been shown that the collision-induced (non-radiative) relaxation is much faster than the radiative one and completely dominates with typical relaxation times ranging between nanoseconds and microseconds. The relaxation always ends up in the Kr + 2 electronic ground state, high electronic excitations survive for longer times than lower excitations due to spin-orbit coupling effects.

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The present work highlights the links between melting properties and structural excitation spectra of small gold and silver clusters. The heat capacity curves are computed for Ag20, Au20, Ag55, Au55 and their ions, using a parallel-tempering molecular dynamics scheme to explore the density functional based tight binding (DFTB) potential energy surfaces and the multiple histogram method. It is found that clusters having very symmetric lowest energy structures (Au20, Ag55 and their ions) present sharp or relatively sharp solid-to-liquid transitions and large melting temperatures, important structural excitation energies and a discrete isomer spectrum. Opposite trends are observed for less ordered clusters (Ag20, Au55 and their ions). Regarding the structural evolution with temperature, very symmetric clusters exhibit minor evolution up to the starting melting temperature. The present study also highlights that, in contrast with the case of Au20, a single electron excess or deficiency is not determinant regarding the melting characteristics, even quantitatively, for clusters containing 55 atoms, for gold as for silver.

The low energy structures of neutral and cationic pyrene clusters containing up to seven molecules are searched through a global exploration scheme combining Parallel Tempering Monte Carlo algorithm and local quenches. The potential energies are computed at the Density Functional based Tight Binding level for neutrals and Configuration-Interaction Density-Functional based Tight Binding for cations in order to treat properly the charge resonance. New simplified versions of these schemes are also presented and used during the global exploration. Neutral clusters are shown to be made of compact assemblies of sub-blocs containing up to three units whereas cations present a charged dimer or trimer core surrounded by neutral units. The structural features of the clusters are analyzed and correlated for the cation with the charge distribution. The stability of clusters is also discussed in terms of cohesive and evaporation energies. Adiabatic and vertical ionization potentials are also discussed.

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¡¡¡¡¡¡¡ HEAD The scopeρ of this article is to present an overview of the Density Functional based Tight Binding (DFTB) method and its applications. The paper introduces the basics of DFTB and its standard formulation up to second order. It also addresses methodological developments such as third order expansion, inclusion of non-covalent interactions, Long-range short range separation to solve the self-interaction error, developments for excited states via the Time-dependent DFTB scheme, inclusion of DFTB in hybrid high-level/low level schemes (DFT/DFTB or DFTB/MM), fragment decomposition of large systems, large scale potential energy landscape exploration with molecular dynamics in ground or excited states, excited states non-adiabatic dynamics. A number of applications are reviewed, focusing on-(i)-the variety of systems that have been studied ======= The scope of this article is to present an overview of the Density Functional based Tight Binding (DFTB) method and its applications. The paper introduces the basics of DFTB and its standard formulation up to second order. It also addresses methodological developments such as third order expansion, inclusion of non-covalent interactions, schemes to solve the self-interaction error, implementation of long-range short-range separation , treatment of excited states via the time-dependent DFTB scheme, inclusion of DFTB in hybrid high-level/low level schemes (DFT/DFTB or DFTB/MM), fragment decomposition of large systems, large scale potential energy landscape exploration with molecular dynamics in ground or excited states, non-adiabatic dynamics. A number of applications are reviewed, focusing on-(i)-the variety of systems that have been studied ¿¿¿¿¿¿¿ f70d2388346dfe2c2221fafac6c2d19fb13b68b9 such as small molecules, large molecules and biomolecules, bare or functionalized clusters, supported or embedded systems, and-(ii)-properties and processes, such as vi-brational spectroscopy, collisions, fragmentation, thermodynamics or non-adiabatic dynamics. Finally outlines and perspectives are given.

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Infrared spectra are computed for neutral and cationic clusters of Polycyclic Aromatic Hydrocarbon clusters, namely (C16H10) (0/+) n=1,4 , using the Density Functional based Tight Binding scheme combined with a Configuration Interaction scheme (DFTB-CI) in the double harmonic approximation. Cross-comparison is carried out with DFT and simple DFTB. Similarly to the monomer cation, the IR spectra of cluster cations are characterized by a depletion of the intensity of the CH stretch modes around 3000 cm −1 , with a weak revival for n = 3 and 4. The in-plane CCC modes in the region 1400-2000 cm −1 are enhanced while the CH bending modes in the range 700-1000 cm −1 are significantly weakened with respect to the monomer cation, in particular for n = 2. Finally, soft modes corresponding to diedral fluctuations of the monomers within the central stack of the ion structure, possibly mixed with monomer folding, are also observed in the region 70-120 cm −1 .

We report threshold collision induced dissociation experiments on cationic pyrene clusters, for sizes n=2 to 6. Fragmentation cross-sections were recorded as a function of the collision energy and analyzed with a statistical model. This model can account for the dissociation cascades and provides values for the dissociation energies. These values, of the order of 0.7 to 1 eV, are in excellent agreement with those previously derived from thermal evaporation. They confirm the charge resonance stability enhancement predicted by theoretical calculations. In addition, a remarkable agreement is obtained with theoretical predictions for the two smaller sizes n=2 and 3. For the larger sizes, the agreement remains good, although the theoretical values obtained for the most stable structures are systematically higher by 0.2 eV. This offset could be attributed to approximations in the calculations. Still, there is indication in the results of an incomplete description of the role of isomerization and/or direct dissociation upon collisions. Finally, by-product clusters containing dehydrogenated species are found to dissociate at energies comparable to the non-dehydrogenated ones, which shows no evidence for covalent bonds within the clusters.

deMon2k is a readily available program specialized in Density Functional Theory (DFT) simulations within the framework of Auxiliary DFT. This article is intended as a tutorial-review of the capabilities of the program for molecular simulations involving ground and excited electronic states. The program implements an additive QM/MM (quantum mechanics/molecular mechanics) module relying either on non-polarizable or polarizable force fields. QM/MM methodologies available in deMon2k include ground-state geometry optimizations, ground-state Born-Oppenheimer molecular dynamics simulations, Ehrenfest non-adiabatic molecular dynamics simulations, and attosecond electron dynamics. In addition several electric and magnetic properties can be computed with QM/MM. We review the framework implemented in the program, including the most recently implemented options (link atoms, implicit continuum for remote environments, metadynamics, etc.), together with six applicative examples. The applications involve (i) a reactivity study of a cyclic organic molecule in water; (ii) the establishment of free-energy profiles for nucleophilic-substitution reactions by the umbrella sampling method; (iii) the construction of two-dimensional free energy maps by metadynamics simulations; (iv) the simulation of UV-visible absorption spectra of a solvated chromophore molecule; (v) the simulation of a free energy profile for an electron transfer reaction within Marcus theory; and (vi) the simulation of fragmentation of a peptide after collision with a high-energy proton.

La modélisation des agrégats moléculaires neutres ou cationiques représente encore un enjeu difficile pour les approches ab initio dès que les molécules impliquées sont de grande taille ou que leur nombre dépasse plusieurs unités. Nous développons, dans cette thèse, une méthode mixte fondée sur la combinaison de la méthode DFTB, une approximation de la Théorie de la Fonctionnelle de la Densité (DFT), avec un schéma d’Interaction de Configurations (CI). Cette méthode (DFTB-CI) présente une approche originale et efficace permettant une description correcte de la résonance de charge au sein des agrégats cationiques. L’application aux agrégats d’hydrocarbures aromatiques polycycliques intéresse plusieurs domaines tels la physico-chimie du milieu interstellaire, la chimie de l’atmosphère ou encore les processus de combustion. Ce travail a permis de caractériser les propriétés structurales de l’état fondamental des agrégats neutres et cationiques de pyrène et de coronène contenant jusqu’à une dizaine de molécules. Il a nécessité la mise en place d’une stratégie multi-méthodes afin de rendre efficace la recherche des structures les plus stables à l’aide de l’algorithme d’exploration globale, « Parallel Tempering Monte- Carlo ». A la suite des propriétés structurales, nous avons déterminé les grandeurs caractérisant la stabilité des agrégats (énergies de cohésion et de dissociation) ainsi que les propriétés électroniques comme la dépendance des potentiels d’ionisation en fonction de la taille, en très bon accord avec les résultats expérimentaux. Finalement, nous avons proposé une extension du modèle DFTB-CI pour calculer les états excités des agrégats moléculaires. Les applications aux dimères sont en bon accord avec les calculs ab initio. Une application aux petits agrégats cationiques de benzène et de pyrène a permis la détermination de leurs spectres électroniques d’absorption.

We present a new set of solar metallicity atmosphere and evolutionary models for very cool brown dwarfs and self-luminous giant exoplanets, which we term ATMO 2020. Atmosphere models are generated with our state-of-the-art 1D radiative-convective equilibrium code ATMO, and are used as surface boundary conditions to calculate the interior structure and evolution of 0.001–0.075 M⊙ objects. Our models include several key improvements to the input physics used in previous models available in the literature. Most notably, the use of a new H–He equation of state including ab initio quantum molecular dynamics calculations has raised the mass by ~1−2% at the stellar–substellar boundary and has altered the cooling tracks around the hydrogen and deuterium burning minimum masses. A second key improvement concerns updated molecular opacities in our atmosphere model ATMO, which now contains significantly more line transitions required to accurately capture the opacity in these hot atmospheres. This leads to warmer atmospheric temperature structures, further changing the cooling curves and predicted emission spectra of substellar objects. We present significant improvement for the treatment of the collisionally broadened potassium resonance doublet, and highlight the importance of these lines in shaping the red-optical and near-infrared spectrum of brown dwarfs. We generate three different grids of model simulations, one using equilibrium chemistry and two using non-equilibrium chemistry due to vertical mixing, all three computed self-consistently with the pressure-temperature structure of the atmosphere. We show the impact of vertical mixing on emission spectra and in colour-magnitude diagrams, highlighting how the 3.5−5.5 μm flux window can be used to calibrate vertical mixing in cool T–Y spectral type objects.

The aim of the present thesis encompasses different processes related to the storage of energy coming from renewable sources. Concretely, this thesis aims to study, from a theoretical point of view, the processes related to the plasma-assisted Sabatier reaction (CO 2 + 4 H2 → CH4 + 2 H2O), where the heterogeneous catalyst is composed by Ni/Ru elements. The research is consequently split in the topics developed at each partner specialties. In the university of Perugia, the plasma/gas phase processes are considered, concretely the study of the OH + H 2 using the quantum-classical method. The main innovative procedure has been to add a long-range potential tail to the already available. Potential Energy Surface (PES), converting it into a suitable one for non reactive processes, while maintaining the accuracy of the ab initio, necessary for the reactive processes. In this sense we carried out a study of OH + H2 scattering using a quantum-classical method, treating quantally vibrations and classically both translations and rotations. The good agreement between the state specific quantum- classical reactive probabilities and the corresponding full quantum ones prompted the extension of the study to state to state probabilities for non reactive vibrational energy exchange. The study showed that H 2 reactive dynamics depends on the vibrational excitation, while the non reactive one is mainly vibrationally adiabatic. On the contrary, OH reactive dynamics is not affected by its vibrational excitation, whereas the non reactive one might produce some pumping up to higher vibrational states. In the university Paul Sabatier of Toulouse, the Ru clusters and nanoparticles, part of the industrial catalyst are studied using the DFTB approach. The intend was to investigate the ability of DFTB to provide reliable results about electronic structure, structural properties and stability of monometallic ruthenium systems covering the size range from small clusters to larger nanoparticles and the bulk. Due to the fact that the electronic bonding and structural organization of ruthenium cluster are somewhat specific in regard of other metal clusters, it is challenging to examine whether DFTB is able to account for such peculiarities. Parallel-Tempering Molecular Dynamics (PTMD), was used in combination with periodic quenching to achieve global optimization of neutral, cationic and anionic clusters in the range n=3-20.[...]

Agrégats moléculaires Infrared spectroscopy HAP Charged system and open shell Dynamique moléculaire SCC-DFTB Electronic Structure PAH Molecular dynamics Free energy surface 2 Electronic structure Atrazine Chimie quantique Fi MOLECULE Barium Anharmonic Infrared Spectroscopy ISM lines and bands Au147 Fenhexamid Configuration interaction Dynamique électronique Biodegradation Charge resonance J 52 LYING ELECTRONIC STATES Carbonaceous grains Finite-temperature effects Brown dwarfs ISM molecules CAH Agrégats protonés Energie renouvelable CONFIGURATION-INTERACTION Catalysis Charge transfer state Ionizing radiation Benzene MS-CASPT2 Line profiles / brown dwarfs Champ de forces Hv 52 EMISSION-SPECTRA Clusters MAGNESIUM DEUTERIDE DFTB Catalyse Fenhexamide Astrochemistry Quantum chemistry Auxiliary density functional theory Clay mineral Argon Dftb 22 pole cryogenic ion trap DUST Dynamique Moléculaire Car-Parrinello Amorphous DFTB-CI Gold Ionisation Mass spectrometry Femtochimie Carbon cluster Laboratory astrophysics Dusty plasma Excited states GALAXIES Car-Parrinello molecular dynamics EXTENDED RED EMISSION Metamitron INFRARED-EMISSION Electron and nuclear dynamics DFT Force fields Collision Induced Dissociation MINI-SYMPOSIUM SPECTROSCOPY OF INTERFACES Effets de température Matrice de gaz rare 1 Density Functional Theory Infrared ISM Molecular clusters Methods laboratory molecular IR EMISSION Modeling Approche mixte quantique/classique Metadynamics Density functional theory Interstellar extinction Chemical shift FAR-IR Ion trap Nanoparticles Argile Interaction de configurations Alanine dipeptide CONSTANTS