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The interaction between Mg and (8-hydroxyquinoline)aluminum, (Alq3), is examined via ab initio molecular dynamics based on density functional theory. The results obtained from two models reveal that bulk calculations provide a better description of the chemical processes involved, allowing the Mg atom to react with two neighboring Alq3 molecules and the core-level shift calculations are in good agreement with experimental measurements only when using the solid phase approach.

The spherical momentum distribution of the protons in ice is extracted from a high resolution deep inelastic neutron scattering experiment. Following a recent path integral Car-Parrinello molecular dynamics study, data were successfully interpreted in terms of an anisotropic Gaussian model, with a statistical accuracy comparable to that of the model independent scheme used previously, but providing more detailed information on the three dimensional potential energy surface experienced by the proton. A recently proposed theoretical concept is also employed to directly calculate the mean force from the experimental neutron Compton profile, and to evaluate the accuracy required to unambiguously resolve and extract the effective proton potential from the experimental data. [ABSTRACT FROM AUTHOR]

A recently proposed approach, called “string method,” allows us to find minimum energy pathways connecting two metastable states of a system [W. E et al., Phys. Rev. B 66, 052301 (2002)]. So far this approach has been only used with empirical force field parametrizations of the atomic potential energy surface or in the context of macroscopic continuum models. Here we show that the string method can be efficiently combined with first-principles molecular dynamics to provide an accurate description of chemical reaction pathways and barriers. We illustrate the first-principles string molecular dynamics by applying it to the study of a surface chemical reaction, for which extensive experimental and theoretical works are available, namely, the adsorption of H2 on the reconstructed Si(100) surface. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a plane-wave ultrasoft pseudopotential implementation of first-principle molecular dynamics, which is well suited to model large molecular systems containing transition metal centers. We describe an efficient strategy for parallelization that includes special features to deal with the augmented charge in the contest of Vanderbilt’s ultrasoft pseudopotentials. We also discuss a simple approach to model molecular systems with a net charge and/or large dipole/quadrupole moments. We present test applications to manganese and iron porphyrins representative of a large class of biologically relevant metalorganic systems. Our results show that accurate density-functional theory calculations on systems with several hundred atoms are feasible with access to moderate computational resources. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have studied the electronic properties of conjugated and saturated dithiol molecules sandwiched between two Au(111) electrodes using first principles density functional calculations with a slab geometry. Relaxation of the molecule/surface adsorption geometry as well as the extended character of the metal electrode states are fully taken into account by our approach. Investigated quantities include the alignment of molecular energy levels with the Fermi energy (E[sub F]) of the metal, the charge transfer and electrostatic potential profile, and the local density of electronic states (LDOS) at E[sub F]. The behavior of the LDOS for benzene–, dibenzene–, and xylyl–dithiol molecules is analyzed and compared with that of alkane–dithiols of various lengths. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We study the binding of molecular oxygen to a graphene sheet and to a (8,0) single walled carbon nanotube, by means of spin-unrestricted density-functional calculations. We find that triplet oxygen retains its spin-polarized state when interacting with graphene or the nanotube. This leads to the formation of a weak bond with essentially no charge transfer between the molecule and the sheet or tube, as one would expect for a physisorptive bond. This result is independent on the approximation used for the exchange-correlation functional. The binding strength, however, depends strongly on the functional, reflecting the inability of current approximation functionals to deal correctly with dispersion forces. Gradient-corrected functionals yield very weak binding at distances around 4 Å, whereas local density functional results yield substantially stronger binding for both graphene and the nanotube at distances of less than 3 Å. The picture of oxygen physisorption is not substantially altered by the presence of topological defects such as 5-7 Stone-Wales pairs. [ABSTRACT FROM AUTHOR]

The structure of the water chain is drawn with alternating orientation of the water molecules to keep the O-H-O bond from bending. The tetrahedral geometry of the water potential and the large dipole create two different water 'phases' for positively and negatively charged electrodes, which is long chains at positive potentials and a honeycomb lattice for negative potentials.

An ab initio molecular dynamics simulation of liquid water has been performed using density functional theory in the Kohn–Sham formulation and a plane wave basis set to determine the electronic structure and the forces at each time step. For an accurate description of the hydrogen bonding in the liquid, it was necessary to extend the exchange functional with a term that depends on the gradient of the electron density. A further important technical detail is that supersoft pseudopotentials were used to treat the valence orbitals of the oxygen atoms in a plane wave expansion. The structural and dynamical properties of the liquid were found to be in good agreement with experiment. The ab initio molecular dynamics also yields information on the electronic structure. The electronic feature of special interest is the lowest unoccupied molecular orbital (LUMO) of the liquid which is the state occupied by a thermalized excess electron in the conductive state. The main result of calculating the liquid LUMO is that it is a delocalized state distributed over interstitial space between the molecules with a significant admixture of the σ* orbitals of the individual water molecules. [ABSTRACT FROM AUTHOR]

The ground state geometries of sulfur clusters S2 to S13 have been calculated using a parameter-free density functional (DF) method, combined with molecular dynamics (MD) and simulated annealing techniques. The results are in good agreement with available experimental data, and should provide reliable predictions where detailed measurements are lacking (n=3,4,5,9). The bonding trends are discussed in detail. The MD-DF approach is particularly valuable in larger clusters, where there are many local energy minima with comparable energies. [ABSTRACT FROM AUTHOR]