A rotating bond order formulation of the atom diatom potential energy surface

Potential energy surface for high-energy N + N2 collisions

Potential energy surface calculations yield physical insight into the structure of intermediates and the dynamics of molecular collisions, and they are the first step toward molecular simulations that provide physical insight into energy transfer, reaction, and dissociation probabilities. The potential energy surface for high-energy collisions of N₂ with N can be used for modeling chemical dynamics and energy transfer in atmospheric shock waves. Here we present an analytic ground-state. (⁴A'') potential energy surface for N₃ that governs electronically adiabatic collisions of N₂(¹Σ+g) with N(⁴S). The fitted surface consists of a pairwise potential based on an accurate diatomic potential energy curve plus a connected permutationally invariant polynomial (PIP) in mixed-exponential-Gaussian bond order variables (MEGs) for the three-body part. The three-body fit is based on multireference complete active space second order perturbation theory (CASPT2) calculations. The quality of the quartet N₃ fit is comparable to that for a previous fit of the NO₂ potential. We characterize two local minima of N₃, two tight transition structures, two van der Waals geometries, and the noncollinear reaction path for the symmetric exchange reaction. The nonreactive approach of an N atom to N₂ along the perpendicular bisector is more repulsive than the collinear reproach, but plots of the force on the bond versus the potential energy at the distance of closest approach allow us to infer that vibrational energy transfer should occur much more readily in high-energy collinear collisions than in high-energy perpendicular-bisector collisions.

Impact of the Long-Range Interaction on the Efficiency of the Li + ClH → LiCl + H Reaction

Quantum and quasiclassical calculations have been performed to compute the low energy efficiency of the Li + ClH → LiCl + H reaction on some potential energy surfaces fitted to ab initio electronic energies using different functional forms. The outcomes of the calculations show marked differences at threshold and in the shape of the excitation function in seeming contrast with the height of the saddle to reaction and the width of the cone of acceptance. The differences in the computed reactive probability and cross section are rationalized in terms of the attractive/repulsive nature of the long-range interaction and the inability of trajectory techniques to deal with threshold effects. The vestiges of these features in the value of the thermal rate coefficients are also commented on.

Closer versus Long Range Interaction Effects on the Non-Arrhenius Behavior of Quasi-Resonant O2 + N2 Collisions

We report in this paper an investigation on energy transfer processes from vibration to vibration and/or translation in thermal and subthermal regimes for the O₂ + N₂ system performed using quantum-classical calculations on different empirical, semiempirical, and ab initio potential energy surfaces. In particular, the paper focuses on the rationalization of the non-Arrhenius behavior (inversion of the temperature dependence) of the quasi-resonant vibration-to-vibration energy transfer transition rate coefficients at threshold. To better understand the microscopic nature of the involved processes, we pushed the calculations to the detail of the related cross sections and analyzed the impact of the medium and long-range components of the interaction on them. Furthermore, the variation with temperature of the dependence of the quasi-resonant rate coefficient on the vibrational energy gap between initial and final vibrational states and the effectiveness of quantum-classical calculations to overcome the limitations of the purely classical treatments were also investigated. These treatments, handled in an open molecular science fashion by chaining data and competencies of the various laboratories using a grid empowered molecular simulator, have allowed a rationalization of the dependence of the computed rate coefficients in terms of the distortion of the O₂-N₂ configuration during the diatom-diatom collisions. A way of relating such distortions to a smooth and continuous progress variable, allowing a proper evolution from both long to closer range formulation of the interaction and from its entrance to exit channel (through the strong interaction region) relaxed graphical representations, is also discussed in the paper.

The shape of the potential energy surface and the thermal rate coefficients of the N + N2 reaction

Full-dimensional quantum time-dependent calculations of the detailed probabilities of the N + N2 reaction have been performed on different potential energy surfaces, initial quantum states, and total angular momentum quantum numbers. The calculations allowed a rationalization of the effect of both moving the saddle to reaction out of collinearity and lowering its height. On some of these surfaces, more extended studies of the reactive dynamics of the system were performed. On one of them also, thermal rate coefficients were computed using J = 0 quantum probabilities and the J-shift model after testing the applicability of such a model against centrifugal sudden results. A comparison of the calculated thermal rate coefficients with theoretical and experimental data available from the literature is also made, and possible effects of inserting an intermediate well at the top of the saddle are argued.

A Nonorthogonal Coordinate Approach to Atom-Diatom Parallel Reactive Scattering Calculations

Computational approaches to the evaluation of the reactive scattering properties of atom- diatom reactions are revisited. The aim is to exploit both the use of nonorthogonal coordinates in reactive scattering and the restructuring of related computer codes for concurrent computing. To this end, bond length, bond-order and hyperspherical bond order coordinate formalisms are examined for the collinear case. At the same time, the evolution of parallel models from coarse to fine granularity and the development of parallelization supports from directive libraries to programming environments are discussed. The scalability of related codes is tested by measuring the performances of restructured codes. The suitability of the use of nonorthogonal coordinates for scattering purposes is tested by performing collinear calculations for the H + H2 reaction.