The minimum-energy pathway and the energetics of the reaction H + O2 ⇌ HO2 ⇌ O + OH have been computed at four levels of density-functional theory, involving local and nonlocal exchange and correlation terms. The four exchange-correlation potentials produce very similar geometries for the HO2 intermediate, and similar trajectories for the reaction path in the neighborhood of HO2; the pathways begin to differ more significantly at greater distances between reactant or product species, as the surface flattens out toward the asymptotic dissociation energy. The experimental energetics are reproduced most accurately by nonlocal exchange plus local correlation. Addition of a nonlocal correlation term worsens the agreement.

The minimum-energy pathway and the energetics of the reaction H + O-2 reversible arrow HO2 reversible arrow O + OH have been computed at four levels of density-functional theory, involving local and nonlocal exchange and correlation terms. The four exchange-correlation potentials produce very similar geometries for the HO2 intermediate, and similar trajectories for the reaction path in the neighborhood of HO2 the pathways begin to differ more significantly at greater distances between reactant or product species, as the surface flattens out toward the asymptotic dissociation energy. The experimental energetics are reproduced most accurately by nonlocal exchange plus local correlation. Addition of a nonlocal correlation term worsens the agreement. (C) 1998 Elsevier Science B.V. All rights reserved.

We describe the implementation of a new grid-free density-functional technique for exchange-correlation potentials of ρ1/3 form (exchange-only local density-functional theory potentials). The potential is fitted to integrable functional forms by solving a set of nonlinear equations, rather than by fitting on a three-dimensional grid of points. This completely analytical method produces smooth energy surfaces and exact energy gradients. The method is found to be several times faster computationally in single-point calculations than a comparable grid-based method with a moderate number of grid points, and it is more than an order of magnitude faster for geometry optimizations. The analytical method is tested on the torsional energy surfaces of the classic isoelectronic series C2H6, N2H4, and H2O2, using the Hartree–Fock–Slater potential (α=2/3). The locations and relative energies of energy extrema, and the structural variations across the potential surfaces, are in good agreement with experimental data and the results of high-quality ab initio studies.

We have implemented a completely analytical linear combination of atomic orbitals-X alpha local density-functional method with exact energy gradients. The superiority of the analytical method over the corresponding grid-based technique in terms of speed, smooth energy surfaces, and accurate gradients is demonstrated in calculations for the umbrella inversion mode of ammonia. The equilibrium geometry and inversion barrier height are also shown to compare well to experimental data.