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We investigate theoretically the progressive coating of C60 by several sodium atoms. Density functional calculations using a nonlocal functional are performed for NaC60 and Na2C60 in various configurations. These data are used to construct an empirical atomistic model in order to treat larger sizes in a statistical and dynamical context. Fluctuating charges are incorporated to account for charge transfer between sodium and carbon atoms. By performing systematic global optimization in the size range 1 ≤ n ≤ 30, we find that NanC60 is homogeneously coated at small sizes, and that a growing droplet is formed above n ≥ 8. The separate effects of single ionization and thermalization are also considered, as well as the changes due to a strong external electric field. The present results are discussed in the light of various experimental data.

We present a study of the stability of n-vacancies (V (n)) and hydrogens in the hexagonal close-packed titanium system computed by means of first-principles calculations. In this work, performed by using the generalized gradient approximation of density functional theory, we focused on the formation energies and the processes of migration of these defects. In the first part, the calculated formation energy of the monovacancy presents a disagreement with experimental data, as already mentioned in the literature. The activation energy is underestimated by almost 20%. The stability of compact divacancies was then studied. We show that a divacancy is more stable than a monovacancy if their migration energies are of the same order of magnitude. We also predict that the migration process in the basal plane of the divacancy is controlled by an intermediate state corresponding to a body-centered triangle (BO site). The case of the trivacancies is finally considered from an energetic point of view. In the second part, the insertion of hydrogen and the processes of its migration are discussed. We obtain a satisfactory agreement with experimental measurements. The chemical nature of the interactions between hydrogen and titanium are discussed, and we show that the H-atom presents an anionic behavior in the metal. The trapping energy of hydrogen in a monovacancy as a function of the number of hydrogen atoms is finally presented.

We report a density-functional study of some properties of the dissociative interaction of hydrogen and oxygen molecules on small palladium clusters (n = 5, 7, and 10). The calculated physisorption and chemisorption energies are compared with those of the infinite (111) palladium surface. First, adsorption of atomic hydrogen and oxygen is investigated on the Pd5, Pd7, and Pd10 clusters. Second, the interaction between H2 (O2) and the small Pd5 cluster is examined and compared to the process occurring on an infinite (111) surface. Finally, the simultaneous adsorption of two hydrogen (oxygen) atoms is analyzed in detail. As shown in a previous work, the binding energy of the first hydrogen (oxygen) atom does not depend significantly on the cluster size, and small two-layer clusters (n ≤ 10) can be used to determine with accuracy the interaction of atomic adsorbates with an infinite (111) palladium surface. In this study, we show that the dissociative chemisorption of H2 and more especially of O2 on a small palladium cluster may lead to erroneous binding energy:  the cluster's size may prevent an accurate description of the adsorbate-adsorbate interaction as a function of their distance. It is demonstrated that a good choice of both the size and the shape of the cluster is preponderant for a good description of the dissociative adsorption of H2 and O2 on an infinite (111) surface.

To investigate the elementary events of the decomposition of transition metal complexes, a density functional study of the systems Cr(C 6 H 6 ) 2 and Cr(C 6 H 6 ) as well as their corresponding cations, has been carried out. The results give a low bond energy (0.36 eV) for the neutral Cr(C 6 H 6 ) compound while the binding energy of the cation Cr(C 6 H 6 ) + is much higher (1.84 eV), in good agreement with previous theoretical and experimental studies. Concerning the dissociation of Cr(C 6 H 6 ) 2 , the first process (dissociation of the first ligand) needs much higher energy than the second one, and depends on the intersystem crossing relaxation (ISC) of the intermediate Cr(C 6 H 6 ) compound. Taking into account the ISC process, the three-body decomposition occurs at 3.40 eV, in good agreement with recent experimental results. In the case of the Cr(C 6 H 6 ) 2 + dissociation, both processes (with and without ISC) may be available from recent experimental works. Our theoretical results reproduce very well the values corresponding to both ways of decomposition.