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Interactions of O and O2 with the Pd(111) surface are studied using spin-density-functional theory with gradient corrections. The investigation of potential energy surface of O atom on Pd(111) shows the face centered cubic site adsorption to be the most favorable. The diffusion barrier to an adjacent hexagonal closed pack site is 0.56 eV. Several adsorption trajectories are calculated for O2 on Pd(111). We find that the molecule dissociation is either direct but strongly activated or precursor mediated with considerably lower barrier. Three precursor states are found and identified according to geometry, energy, and vibrational frequency. Two precursor states are energetically degenerate with only slightly different geometries. Density difference analysis reveals that the electronic structure of both the molecule and the nearest Pd atoms is modified during the adsorption. However, according to density difference analysis the molecules in the precursor states are neutral and the interaction between O2 and ...

Ab initio Car–Parrinello molecular dynamics simulations of a Li+ ion in water have been carried out using the density-functional theory with Becke–Lee–Yang–Parr (BLYP) functional and ultrasoft Vanderbildt pseudopotentials. Both structural and dynamical properties of Li+ have been studied in detail and compared with available neutron scattering and spectroscopic data. Excellent agreement is obtained with the existing experimental data for the structure of the first hydration shell around the Li+ ion. Spectral features of bound water are compared to those of bulk water. Reasonable agreement is obtained with IR and Raman experiments. The ab initio simulation results have also been used to derive a Li+–water interaction potential. The best fit of the data gave a simple single-exponential potential function, which reproduces very well the liquid structure from the original ab initio simulations. This potential model, together with the simple point charge (SPC) water model, was applied to calculate the hydratio...

Grand canonical molecular dynamics simulations have been performed to determine the sizes of critical nuclei in gas–liquid nucleation. The studied system consists of Lennard-Jones (LJ) argon atoms with a potential cutoff of 4.9σ at a reduced temperature of 0.694 and at vapor supersaturations between 3.5 and 6.2. To facilitate comparison with nucleation theories, we have also determined the equilibrium vapor pressure of LJ argon as a function of temperature. The results are compared with other simulation studies, and with predictions of the classical (CNT) and density functional (DFT) nucleation theories. We find that the semiempirical version of the DFT is in excellent agreement with the simulation results, and that the CNT underestimates the number of LJ atoms in the nuclei only very slightly.

Sulphuric acid has a tendency to form hydrates, small clusters containing a few water molecules, in the gas phase. Hydrate formation has a stabilising effect on the vapour as the pressure of sulphuric acid drops (relative to unhydrated vapor), decreasing the nucleation rate. In classical nucleation theories the hydration energies and the hydrate distribution are predicted assuming that hydrates can be described as liquid droplets having thermodynamic properties of bulk liquid. To obtain a better understanding of the structures and formation energies of the smallest clusters, we have performed ab initio density functional calculations of the mono-,di-, and trihydrates. The hydrogen bonds between the molecules are found to be strong. The more water molecules the hydrate contains, the clearer ring-like structure is formed. Comparison to classical values for the hydration enthalpies confirms that the properties of bulk liquid do not describe the properties of the smallest clusters too well. The energy barrier...

The structures of an isolated CAl12 cluster and a solid composed of CAl12 clusters have been studied using the Car–Parrinello method, based on the density functional theory and the local density approximation. We have compared the results of using the ultrasoft Vanderbilt pseudopotential with those of both a traditional pseudopotential and a linear combination of atomic orbitals method. We have confirmed the high stability of the cluster in its icosahedral structure. However, we show that the cluster‐assembled solid is unstable against melting of the clusters, as previously found for SiAl12.

Charge defects in water created by excess or missing protons appear in the form of solvated hydronium H3O+ and hydroxyl OH− ions. Using the method of ab initio molecular dynamics, we have investigated the structure and proton transfer dynamics of the solvation complexes, which embed the ions in the network of hydrogen bonds in the liquid. In our ab initio molecular dynamics approach, the interatomic forces are calculated each time step from the instantaneous electronic structure using density functional methods. All hydrogen atoms, including the excess proton, are treated as classical particles with the mass of a deuterium atom. For the H3O+ ion we find a dynamic solvation complex, which continuously fluctuates between a (H5O2)+ and a (H9O4)+ structure as a result of proton transfer. The OH− has a predominantly planar fourfold coordination forming a (H9O5)− complex. Occasionally this complex is transformed in a more open tetrahedral (H7O4)− structure. Proton transfer is observed only for the more waterlike (H7O4)− complex. Transport of the charge defects is a concerted dynamical process coupling proton transfer along hydrogen bonds and reorganization of the local environment. The simulation results strongly support the structural diffusion mechanism for charge transport. In this model, the entire structure—and not the constituent particles—of the charged complex migrates through the hydrogen bond network. For H3O+, we propose that transport of the excess proton is driven by coordination fluctuations in the first solvation shell (i.e., second solvation shell dynamics). The rate‐limiting step for OH− diffusion is the formation of the (H7O4)− structure, which is the solvation state showing proton transfer activity.

We report results from spin-polarized ab initio local spin-density calculations for the silicon vacancy (VSi) in 3C– and 2H–SiC in all its possible charge states. The calculated electronic structure for SiC reveals the presence of a stable spin-aligned electron-state t2 near the midgap. The neutral and doubly negative charge states of VSi in 3C–SiC are stabilized in a high-spin configuration with S=1 giving rise to a ground state, which is a many-electron orbital singlet 3T1. For the singly negative VSi, we find a high-spin ground-state4A2 with S=3/2. In the high-spin configuration, VSi preserves the Td symmetry. These results indicate that in neutral, singly, and doubly negative charge states a strong exchange coupling, which prefers parallel electron spins, overcomes the Jahn–Teller energy. In other charge states, the ground state of VSi has a low-spin configuration.