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We present an efficient, analytical, and simple route to approximating tunneling splittings in multidimensional chemical systems, directly from ab initio computations. The method is based on the Wentzel–Kramers–Brillouin (WKB) approximation combined with the vibrational perturbation theory. Anharmonicity and corner-cutting effects are implicitly accounted for without requiring a full potential energy surface. We test this method on the following three systems: a model one-dimensional double-well potential, the isomerization of malonaldehyde, and the isomerization of tropolone. The method is shown to be efficient and reliable.

Tunnelling controlled chemical reactions are those which preferably proceed through pathways with high but narrow potential energy barriers, via quantum tunnelling, resulting in a product that would be disfavoured classically. These reactions are very sensitive to barrier width, height and temperature and so dynamical theoretical methods are required to describe these processes. Recent experimental work on charge-tagged phenyl pyruvic acid derivatives has found, in contrast to similar systems, no evidence of tunnelling control. Using semiclassical transition state theory, we rationalise these results and find tunnelling is significant in this system.

The reaction O2(ν) + O2(ν = 0) → O3(X1A1) + O(3P) is studied by quantum time-dependent and time-independent methods, for high vibrational excitation of one of the reactants. State-selected reaction rate constants are computed showing evidence of reaction but their values are too small to explain previous experimental measurements. © 1997 Elsevier Science B.V. All rights reserved.

The torsional path integral Monte Carlo (TPIMC) technique is used to study the five lowest-energy conformers of glycine. The theoretical method provides an anharmonic and quantum-mechanical description of conformational free energy and is used for the first time with an ab initio potential energy surface. The 3-dimensional torsional potential energy surface of glycine was obtained at the MP2/6-311++G** level of theory and is optimized with respect to the non-torsional degrees of freedom. Calculated conformer populations compare well with those reported in recent matrix-isolation infrared spectroscopy experiments. An additional conformer, not yet observed, is predicted to be heavily populated in the thermal equilibria probed by experiment, and a new explanation for its elusiveness is provided. Quantum effects, such as zero point energy, are found to substantially alter conformer populations, and an algorithm for estimating the role of non-torsional vibrations in the conformational thermodynamics of a molecule is introduced.

We describe classical-trajectory calculations of sputtering yields for Ar+-ion collisions with a Si(001) surface. The Ar+-Si and short-ranged Si-Si interaction potentials were calculated using the ab initio Hartree-Fock and configuration-interaction methods. The low-energy potential describing the silicon solid is the two- and three-body form due to Stillinger and Weber. We compare the calculated sputtering yields with experiment. The potential-energy surface strongly influences the calculated sputtering yields, and it is found that the most reasonable agreement is obtained from our potentials using the (2×1) Si(001) reconstructed surface rather than the bulk-terminated surface. Analysis of the kinetic energy and angular distributions of the sputtered silicon atoms and of cluster yields has provided a mechanism of ejection. © 1989 The American Physical Society.

One hundred years ago, Niels Bohr's pioneering paper on the electronic structure of the hydrogen atom revolutionized atomic theory.

The title reaction is an important step in the synthesis of ammonia over ruthenium-based catalysts, which have recently received increasing attention. The study presented here is fully theoretical and uses a reduced dimensionality approach to the surface reaction. A potential energy surface is first obtained by density functional calculations using a supercell approach. We have then calculated rate constants in the range of temperature 200-1000 K, which includes the temperature at which the ammonia production is usually performed. This has been done within the framework of variational transition-state theory in a reaction-path formulation, comparing different levels of theory and including quantum transmission and reflection effects by semiclassical methods. Calculated rates are fitted by the Arrhenius law. Finally, we have carried out an accurate investigation of the isotopic effect by replacing hydrogen with deuterium.

We propose a mechanism of photostructural changes in amorphous As2S3 ( a-As2S3) on the basis of ab initio molecular orbital calculations on clusters of atoms modeling the local structure of the amorphous system. We have found that trigonal AsS3 pyramidal units can be transformed into a fivefold coordinated As site having four As-S bonds and one As-As bond via a photoionization process. This photoinduced coordination defect center exhibits a lower photoabsorption energy as compared with the usual pyramidal structure, explaining the observed photodarkening effect of a-As2S3.