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The d-band model has proven to be effective for understanding trends in the chemisorption of various adsorbates on transition metal surfaces. However, hydrogen adsorption at the atop site of transition metals and their bimetallic alloy surfaces do not always correlate well with the d-band center of the adsorption site. Additionally, the d-band model cannot explain the disappearance of the local minima for H adsorption at the hollow site on the potential energy surface of 5d single-atom element doped Au and Ag(111) surfaces. Here, we use a simple model with factors, including the d-band center, filling of the d-band, renormalized adsorbate states, coupling matrix elements, and surface–adsorbate bond lengths, to correlate with the density functional theory calculated H binding energies on both mono- and bimetallic (111) surfaces. Our results suggest that H adsorption at metal-atop sites is determined by all these factors, not only by the d-band center. The strong adsorption of H at the atop sites of 5d metal surfaces can be explained by their lower repulsive contribution. [ABSTRACT FROM AUTHOR]

The harmonic approximation to transition state theory simplifies the problem of calculating a chemical reaction rate to identifying relevant low energy saddle points in a chemical system. Here, we present a saddle point finding method which does not require knowledge of specific product states. In the method, the potential energy landscape is transformed into the square of the gradient, which converts all critical points of the original potential energy surface into global minima. A biasing term is added to the gradient squared landscape to stabilize the low energy saddle points near a minimum of interest, and destabilize other critical points. We demonstrate that this method is competitive with the dimer min-mode following method in terms of the number of force evaluations required to find a set of low-energy saddle points around a reactant minimum. [ABSTRACT FROM AUTHOR]

The dimer method is a minimum mode following algorithm for finding saddle points on a potential energy surface of atomic systems. Here, the dimer method is extended to include the cell degrees of freedom for periodic solid-state systems. Using this method, reaction pathways of solid-solid phase transitions can be determined without having to specify the final state structure or reaction mechanism. Example calculations include concerted phase transitions between CdSe polymorphs and a nucleation and growth mechanism for the A15 to BCC transition inMo. [ABSTRACT FROM AUTHOR]

The development of transferable interatomic potentials for the diffusion of hydrogen on palladium surfaces can be of significant value for performing molecular simulations. These molecular simulations can, in turn, lead to a better understanding of palladium-hydrogen interactions at the atomic scale. Here, we have built upon previous work to develop an analytical palladium-hydrogen-embedded atom method (EAM) potential to better describe the potential energy surface for hydrogen on palladium surfaces. This EAM potential reproduces minima and transition states calculated with density functional theory for hydrogen on Pd(111) and Pd(110) surfaces. Additionally, this potential was tested by simulating the long timescale dynamics of hydrogen adsorbed on Pd(111). Our simulations show a barrier of ca. 0.49 eV for hydrogen diffusion into the bulk of Pd(111), which is consistent with experimental results. [ABSTRACT FROM AUTHOR]

The results of theoretical calculations of associative desorption of CH4 and H2 from the Ni(111) surface are presented. Both minimum-energy paths and classical dynamics trajectories were generated using density-functional theory to estimate the energy and atomic forces. In particular, the recombination of a subsurface H atom with adsorbed CH3 (methyl) or H at the surface was studied. The calculations do not show any evidence for enhanced CH4 formation as the H atom emerges from the subsurface site. In fact, there is no minimum-energy path for such a concerted process on the energy surface. Dynamical trajectories started at the transition state for the H-atom hop from subsurface to surface site also did not lead to direct formation of a methane molecule but rather led to the formation of a thermally excited H atom and CH3 group bound to the surface. The formation (as well as rupture) of the H–H and C–H bonds only occurs on the exposed side of a surface Ni atom. The transition states are quite similar for the two molecules, except that in the case of the C–H bond, the underlying Ni atom rises out of the surface plane by 0.25 Å. Classical dynamics trajectories started at the transition state for desorption of CH4 show that 15% of the barrier energy, 0.8 eV, is taken up by Ni atom vibrations, while about 60% goes into translation and 20% into vibration of a desorbing CH4 molecule. The most important vibrational modes, accounting for 90% of the vibrational energy, are the four high-frequency CH4 stretches. By time reversibility of the classical trajectories, this means that translational energy is most effective for dissociative adsorption at low-energy characteristic of thermal excitations but energy in stretching modes is also important. Quantum-mechanical tunneling in CH4 dissociative adsorption and associative desorption is estimated to be important below 200 K and is, therefore, not expected to play an important role under typical conditions. An unexpected mechanism for the rotation of the adsorbed methyl group was discovered and illustrated a strong three-center C–H–Ni contribution to the methyl-surface bonding. [ABSTRACT FROM AUTHOR]

A recent letter to the editor (Quapp and Bofill, J Comput Chem 2010, 31, 2526) claims that the nudged elastic band (NEB) method can converge toward gradient extremal paths and not to steepest descent paths, as has been assumed. Here, we show that the NEB does in fact converge to steepest descent paths and that the observed tendency for the NEB to approach gradient extremal paths was a consequence of implementation errors. We also note that while the NEB finds steepest descent paths, these are not necessarily minimum energy paths in the sense of being a set of points which are minima in the potential energy surface perpendicular to the path. An example is given where segments of steepest descent paths follow potential energy ridges. © 2011 Wiley Periodicals, Inc. J Comput Chem , 2011. [ABSTRACT FROM AUTHOR]