Using Hessian update formulae to construct modified Shepard interpolated potential energy surfaces: Application to vibrating surface atoms

Modified Shepard interpolation based on second order Taylor series expansions has proven to be a flexible tool for constructing potential energy surfaces in a range of situations. Extending this to gas-surface dynamics where surface atoms are allowed to move represents a substantial increase in the dimensionality of the problem, reflected in a dramatic increase in the computational cost of the required Hessian (matrix of second derivatives) evaluations. This work demonstrates that using approximate Hessians derived from well known Hessian update formulae and a single accurate Hessian can provide an effective way to avoid this expensive accurate Hessian determination.This work was supported by the NCI National Facility at the ANU

Fast, scalable master equation solution algorithms. III. Direct time propagation accelerated by a diffusion approximation preconditioned iterative solver

In this paper we propose a novel fast and linearly scalable method for solving master equations arising in the context of gas-phase reactive systems, based on an existent stiff ordinary differential equation integrator. The required solution of a linear system involving the Jacobian matrix is achieved using the GMRES iteration preconditioned using the diffusion approximation to the master equation. In this way we avoid the cubic scaling of traditional master equationsolution methods and maintain the low temperature robustness of numerical integration. The method is tested using a master equation modelling the formation of propargyl from the reaction of singlet methylene with acetylene, proceeding through long lived isomerizing intermediates

Modified Shepard interpolation of gas-surface potential energy surfaces with strict plane group symmetry and translational periodicity

A new formulation of modified Shepard interpolation of potential energy surface data for gas-surface reactions has been developed. The approach has been formulated for monoatomic or polyatomic adsorbates interacting with crystalline solid surfaces of any plane group symmetry. The interpolation obeys the two dimensional translational periodicity and plane group symmetry of the solid surface by construction. The interpolation remains continuous and smooth everywhere. The interpolation developed here is suitable for constructing potential energy surfaces by sampling classical trajectories using the Grow procedure. A model function has been used to demonstrate the method, showing the convergence of the classical gas-surface reaction probability

The dynamics of the H₂+CO⁺ reaction on an interpolated potential energy surface

A potential energy surface that describes the title reaction has been constructed by interpolation of ab initio data. Classical trajectory studies on this surface show that the total reaction rate is close to that predicted by a Langevin model, although the mechanism is more complicated than simple ion-molecule capture. Only the HCO⁺ + H product is observed classically. An estimate of the magnitude of rotational inelastic scattering is also reported.financial support from the Ministry of Science, Research and Technology of Iran, and from Shiraz University

Fast, scalable master equation solution algorithms. IV. Lanczos iteration with diffusion approximation preconditioned iterative inversion

In this paper we propose a second linearly scalable method for solving large master equations arising in the context of gas-phase reactive systems. The new method is based on the well-known shift-invert Lanczos iteration using the GMRES iteration preconditioned using the diffusion approximation to the master equation to provide the inverse of the master equation matrix. In this way we avoid the cubic scaling of traditional master equation solution methods while maintaining the speed of a partial spectral decomposition. The method is tested using a master equation modeling the formation of propargyl from the reaction of singlet methylene with acetylene, proceeding through long-lived isomerizing intermediates. (C) 2003 American Institute of Physics

Critical dehydrogenation steps of perhydro-N-ethylcarbazole on Ru(0001) surface

Understanding of the critical atomistic steps during the dehydrogenation process of liquid organic hydrogen carriers (LOHCs) is important to the design of cost-efficient, high-performance LOHC catalysts. Based on the density functional theory (DFT) we studied the thermodynamics and kinetics of the complete dehydrogenation path of perhydro-N-ethylcarbazole (12H-NEC) on Ru(0001) surface, involving the adsorption of 12H-NEC, the discharge of H ions onto Ru surface, and the desorption of H2 and hydrogen-lean NEC. It was found that the bonding of nH-NEC is significantly strengthened for n $\le$ 4 because of the flat aromatic ring. Although the whole dehydrogenation process is endothermic, the release of H from nH-NEC, with H adsorbed onto the Ru surface, was found to be exothermic. The desorption of flat, hydrogen-lean NEC, which costs ~255 kJ/mol, was identified as the most energy demanding step. In addition, the effect of surface morphology on adsorption was studied based on an amorphous surface model. Overall, the results imply more efficient dehydrogenation could be achieved from relatively weak bonding of NEC to catalysts, either through engineering catalyst surface (such as surface defects or smaller catalyst particles) or different catalyst materials. Our calculations also revealed possible dealkylation at elevated temperatures

Trans-Regime Structural Transition of (In3+ + Nb5+) Co-Doped Anatase TiO2 Nanocrystals under High Pressure

Chemical co-doping and high pressure reactions have been broadly used to synthesize novel materials or to tune the physicochemical properties of traditional materials. Here, we take In3+ and Nb5+ ions co-doped anatase TiO2 nanocrystals as an example and report that a combination of both a chemical and a high pressure reaction route is more powerful for the preparation of metastable polymorphs. It is experimentally demonstrated that In3+ and Nb5+ co-doping significantly changes the high-pressure reaction behaviors of anatase TiO2 nanocrystals (<10 nm) and leads to their trans-regime structural transition in terms of in situ Raman analysis, from an anatase to a baddeleyite-like phase under compressive pressures and then to an α-PbO2-like structure under decompressive pressures. This abnormal phase transition is attributed to a defect-induced heterogeneous nucleation mechanism. Furthermore, the stiffness of co-doped TiO2 nanocrystals is significantly enhanced due to the synergistic effects of co-dopants. This research not only proposes a potentially effective strategy to synthesize co-doped metastable polymorphic phases but also suggests one feasible method to improve the mechanical properties of anatase TiO2 nanocrystals

Experimental and theoretical study of rotationally inelastic diffraction of H_2(D_2) from methyl-terminated Si(111)

Fundamental details concerning the interaction between H_2 and CH_3–Si(111) have been elucidated by the combination of diffractive scattering experiments and electronic structure and scattering calculations. Rotationally inelastic diffraction (RID) of H_2 and D_2 from this model hydrocarbon-decorated semiconductor interface has been confirmed for the first time via both time-of-flight and diffraction measurements, with modest j = 0 → 2 RID intensities for H_2 compared to the strong RID features observed for D_2 over a large range of kinematic scattering conditions along two high-symmetry azimuthal directions. The Debye-Waller model was applied to the thermal attenuation of diffraction peaks, allowing for precise determination of the RID probabilities by accounting for incoherent motion of the CH_3–Si(111) surface atoms. The probabilities of rotationally inelastic diffraction of H_2 and D_2 have been quantitatively evaluated as a function of beam energy and scattering angle, and have been compared with complementary electronic structure and scattering calculations to provide insight into the interaction potential between H_2 (D_2) and hence the surface charge density distribution. Specifically, a six-dimensional potential energy surface (PES), describing the electronic structure of the H_2(D_2)/CH_3−Si(111) system, has been computed based on interpolation of density functional theory energies. Quantum and classical dynamics simulations have allowed for an assessment of the accuracy of the PES, and subsequently for identification of the features of the PES that serve as classical turning points. A close scrutiny of the PES reveals the highly anisotropic character of the interaction potential at these turning points. This combination of experiment and theory provides new and important details about the interaction of H_2 with a hybrid organic-semiconductor interface, which can be used to further investigate energy flow in technologically relevant systems

Experimental and theoretical study of rotationally inelastic diffraction of H_2(D_2) from methyl-terminated Si(111)

Fundamental details concerning the interaction between H_2 and CH_3–Si(111) have been elucidated by the combination of diffractive scattering experiments and electronic structure and scattering calculations. Rotationally inelastic diffraction (RID) of H_2 and D_2 from this model hydrocarbon-decorated semiconductor interface has been confirmed for the first time via both time-of-flight and diffraction measurements, with modest j = 0 → 2 RID intensities for H_2 compared to the strong RID features observed for D_2 over a large range of kinematic scattering conditions along two high-symmetry azimuthal directions. The Debye-Waller model was applied to the thermal attenuation of diffraction peaks, allowing for precise determination of the RID probabilities by accounting for incoherent motion of the CH_3–Si(111) surface atoms. The probabilities of rotationally inelastic diffraction of H_2 and D_2 have been quantitatively evaluated as a function of beam energy and scattering angle, and have been compared with complementary electronic structure and scattering calculations to provide insight into the interaction potential between H_2 (D_2) and hence the surface charge density distribution. Specifically, a six-dimensional potential energy surface (PES), describing the electronic structure of the H_2(D_2)/CH_3−Si(111) system, has been computed based on interpolation of density functional theory energies. Quantum and classical dynamics simulations have allowed for an assessment of the accuracy of the PES, and subsequently for identification of the features of the PES that serve as classical turning points. A close scrutiny of the PES reveals the highly anisotropic character of the interaction potential at these turning points. This combination of experiment and theory provides new and important details about the interaction of H_2 with a hybrid organic-semiconductor interface, which can be used to further investigate energy flow in technologically relevant systems

DFT Study of Spin States on Bare and Partially Hydrogenated Si(111) and Si(100) Surfaces

Density functional theory calculations have been performed on slabs of crystalline silicon. The unreconstructed (111) and 2 × 1 reconstructed (100) surfaces have been studied. Starting from surfaces wholly passivated with hydrogen, selected sites were unpassivated to yield small clusters of unpaired spins corresponding to dangling bonds. A range of spin densities were converged for each cluster of dangling bonds to investigate the energetics of different spin patterns in these isolated clusters of dangling bonds. The results were broadly consistent with antiferromagnetic coupling between the spins, only mildly affected by the geometric and electronic effects of the surrounding surface bond passivation