We report the first six-dimensional quantum dynamical calculations of dissociative adsorption and associative desorption. Using a potential energy surface obtained by density functional theory calculations, we show that the initial decrease of the sticking probability with increasing kinetic energy in the system H2/Pd (100), which is usually attributed to the existence of a molecular adsorption state, is due to dynamical steering. In addition, we examine the influence of rotational motion and orientation of the hydrogen molecule on adsorption and desorption.

Powder diffraction techniques are becoming increasingly popular as tools for the determination of crystal structures. The authors of this paper have developed a software package, named PowderSolve, to solve crystal structures from experimental powder diffraction patterns and have applied this package to solve the crystal structures of organic compounds with up to 18 variable degrees of freedom (de®ned in terms of the positions, orientations, and internal torsions of the molecular fragments in the asymmetric unit). The package employs a combination of simulated annealing and rigid-body Rietveld re®nement techniques to maximize the agreement between calculated and experimental powder diffraction patterns. The agreement is measured by a full-pro®le comparison (using the R factor R wp ). As an additional check at the end of the structure solution process, accurate force-®eld energies may be used to con®rm the stability of the proposed structure solutions. To generate the calculated powder diffraction pattern, lattice parameters, peak shape parameters and background parameters must be determined accurately before proceeding with the structure solution calculations. For this purpose, a novel variant of the Pawley algorithm is proposed, which avoids the instabilities of the original Pawley method. The successful application and performance of PowderSolve for crystal structure solution of 14 organic compounds of differing complexity are discussed.

The potential energy surface (PES) of dissociative adsorption of H2 on Pd(100) is investigated using density functional theory and the full-potential linear augmented plane wave (FP-LAPW) method. Several dissociation pathways are identified which have a vanishing energy barrier. A pronounced dependence of the potential energy on "cartwheel" rotations of the molecular axis is found. The calculated PES shows no indication of the presence of a precursor state in front of the surface. Both results indicate that steering effects determine the observed decrease of the sticking coefficient at low energies of the H2 molecules. We show that the topology of the PES is related to the dependence of the covalent H(s)-Pd(d) interactions on the orientation of the H2 molecule.

Following the approach of Yu, Singh, and Krakauer [Phys. Rev. B 43 (1991) 6411] we extended the linearized augmented plane wave code WIEN of Blaha, Schwarz, and coworkers by the evaluation of forces. In this paper we describe the approach, demonstrate the high accuracy of the force calculation, and use them for an efficient geometry optimization of poly-atomic systems.

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