Melting of the Au20 gold cluster : does charge matter?

We investigate the dependence upon charge of the heat capacities of the magic gold cluster Au20 obtained from density functional based tight binding theory within parallel tempering molecular dynamics and the multiple histogram method. The melting temperatures, determined from heat capacity curves, are found to be 1102 K for neutral Au20 and only 866 and 826 K for Au{20} cations and anions respectively. The present work proves that a single charge quantitatively affects the thermal properties of the twentymer even for a global property such as melting

Modeling Charge Resonance in Cationic Molecular Clusters: Combining DFT-Tight Binding with Configuration Interaction

International audienceIn order to investigate charge resonance situations in molecular complexes, Wu et al. (J. Chem. Phys. 2007, 127, 164119) recently proposed a configuration interaction method with a valence bond-like multiconfigurational basis obtained from constrained DFT calculations. We adapt this method to the Self-Consistent Charge Density-Functional-based Tight Binding (SCC-DFTB) approach and provide expressions for the gradients of the energy with respect to the nuclear coordinates. It is shown that the method corrects the wrong SCC-DFTB behavior of the potential energy surface in the dissociation regions. This scheme is applied to determine the structural and stability properties of positively charged molecular dimers with full structural optimization, namely, the benzene dimer cation and the water dimer cation. The method yields binding energies in good agreement with experimental data and high-level reference calculations

Non-adiabatic molecular dynamics investigation of the size dependence of the electronic relaxation in polyacenes

International audienceThe 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

Transition State Spectroscopy of the Photoinduced Ca + CH3F Reaction. 2. Experimental and Ab Initio Studies of the Free Ca***FCH3 Complex

International audienceThe Ca* + CH3F CaF* + CH3 reaction was photoinduced in 1:1 Ca***CH3F complexes formed in a supersonic expansion. The transition state of the reaction was explored by monitoring the electronically excited product, CaF, while scanning the laser that turns on the reaction. Moreover, the electronic structure of the Ca***FCH3 system was studied using ab initio methods by associating a pseudopotential description of the [Ca2+] and [F7+] cores, a core polarization operator on calcium, an extensive Gaussian basis and a treatment of the electronic problem at the CCSD(T) (ground state) and RSPT2 (excited states) level. In this contribution we present experimental results for the free complex and a comparison with the results of a previous experiment where the Ca***CH3F complexes are deposited at the surface of large argon clusters. The ab initio calculations allowed an interpretation of the experimental data in terms of two reaction mechanisms, one involving a partial charge transfer state, the other involving the excitation of the C-F stretch in the CH3F moiety prior to charge transfer

Modelling singly ionized coronene clusters

International audienceWe introduce a model combining a density functional based tight-binding method with a configuration interaction like scheme to treat charge delocalization in ionized molecular clusters. As an application, we determine the size-evolution of some properties of stack coronene clusters, namely charge delocalization, binding energies and ionization potentials

Influence of electronic excitations on the thermodynamics of small carbon clusters: the example of C-3(+)

13th International Symposium on Small Paticles and Inorganic Clusters. - Goteborg, SWEDEN, JUL 23-28, 2006International audienc