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This work presents a study of the thermal evaporation and stability of pyrene (C 16 H 10) n clusters. Thermal evaporation rates of positively charged mass-selected clusters are measured for sizes in the range n=3-40 pyrene units. The experimental setup consists of a gas aggregation source, a thermalization chamber and a time of flight mass spectrometer. A microcanonical Phase Space Theory (PST) simulation is used to determine the dissociation energies of pyrene clusters by fitting the experimental breakdown curves. Calculations using the Density Functional based Tight Binding combined with Configuration Interaction (CI-DFTB) model and a hierarchical optimization scheme are also performed in the range n=2-7 to determine the harmonic frequencies and a theoretical estimation of the dissociation energies. The frequencies are used in the calculations of the density of states needed in the PST simulations, assuming an extrapolation scheme for clusters larger than 7 units. Using the PST model with a minimal set of adjustable parameters, we obtain good fits of the experimental breakdown curves over the full studied size range. The approximations inherent to the PST simulation and the influence of the used parameters are carefully estimated. The derived dissociation energies show significant variations over the studied size range. Compared to neutral clusters, significantly higher values of the dissociation energies are obtained for the smaller sizes and attributed to charge resonance in line with CI-DFTB calculations.

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A benchmark set of relevant geometries of a model protein, the N-acetylphenylalanylamide, is presented to assess the validity of the approximate second-order coupled cluster (CC2) method in studying low-lying excited states of such bio-relevant systems. The studies comprise investigations of basis-set dependence as well as comparison with two multireference methods, the multistate complete active space 2nd order perturbation theory (MS-CASPT2) and the multireference difference dedicated configuration interaction (DDCI) methods. First of all, the applicability and the accuracy of the quasi-linear multireference difference dedicated configuration interaction method have been demonstrated on bio-relevant systems by comparison with the results obtained by the standard MS-CASPT2. Second, both the nature and excitation energy of the first low-lying excited state obtained at the CC2 level are very close to the Davidson corrected CAS+DDCI ones, the mean absolute deviation on the excitation energy being equal to 0.1 eV with a maximum of less than 0.2 eV. Finally, for the following low-lying excited states, if the nature is always well reproduced at the CC2 level, the differences on excitation energies become more important and can depend on the geometry.

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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.

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Even if the ISM remains a cold environment with only few energy available, dynamics effects cannot be neglected since the surface state may be significantly altered due to thermal energy dissipation. It is possible to carry electronic structure calculations on reduced and distorted structures that may reproduce somehow a thermally relaxed surface. Classical molecular dynamics based on a semi-empirical potential remains a method of choice to account for explicit dynamical effects and for large scale surfaces. Within the classical description of the intermolecular forces, only physisorption is accessible. This limitation can be overcome through the combination of dynamics/force field simulations and Self-consistent charge density functional tight binding (SCC-DFTB) calculations. This will be illustrated in the case of the adsorption of PAHs on crystalline and amorphous ices. We will present a complete description of PAH-ice interaction in the ground electronic state at low temperature, providing the binding energies and barrier heights necessary to the on-going improvement of astrochemical models.

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The opacity of alkali atoms, most importantly of Na and K, plays a crucial role in the atmospheres of brown dwarfs and exoplanets. We present a comprehensive study of Na-H 2 collisional profiles at temperatures from 500 to 3000 K, the temperatures prevailing in the atmosphere of brown dwarfs and Jupiter-mass planets. The relevant H 2 perturber densities reach several 10 19 cm −3 in hot (T eff 1500 K) Jupiter-mass planets and can exceed 10 20 cm −3 for more massive or cooler objects. Accurate pressure-broadened profiles that are valid at high densities of H 2 should be incorporated into spectral models. Unified profiles of sodium perturbed by molecular hydrogen were calculated in the semi-classical approach using up-to-date molecular data. New Na-H 2 collisional profiles and their effects on the synthetic spectra of brown dwarfs and hot Jupiters computed with petitCODE are presented.

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This work aims at studying the influence of structural parameters on the computations of 93 Nb quadrupolar interaction and chemical shift parameters in various niobates using first-principles approaches. We demonstrate that some of the computed NMR parameters, especially the isotropic chemical shift and the quadrupolar coupling constant, may differ either the X-ray crystal structure or a relaxed structure are used for the calculation of the spectroscopic properties.

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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.

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