Dressed Adiabatic and Diabatic Potentials To Study Topological Effects for F + H₂

Abstract

We report here on the two lowest, rigorous-accurate diabatic potential energy surfaces (PES), for the F + H2 system, as calculated by including the two dominant topological effects of this system at the low energy region, namely, the Jahn-Teller effect and the Renner-Teller effect. Both effects were treated in the most rigorous way as demanded by the Born-Oppenheimer approach. No approximations were made, and in those cases where convergence was required, it was satisfied. In other words, convergence was attained in all situations. The numerical part that includes the calculation of the two lowest ab initio adiabatic PESs and the corresponding nonadiabatic coupling terms (NACTs) was carried out using the MOLPRO program. The required diabatic potentials are calculated by employing these ab initio adiabatic PESs and the corresponding adiabatic-to-diabatic angles as obtained employing the above-mentioned ab initio NACTs. The relevance of these Renner-Teller/Jahn-Teller diabatic potentials is studied by comparing the dressed-lowest ab initio adiabatic PES and the one formed by diagonalizing the dressed-diabatic 2 × 2 potential matrix. The dressed-potentials are calculated employing the vib-rotational manifold derived for each of the three surfaces, namely, the lowest adiabatic potential and the two diabatic ones. This kind of study was recently recommended by Lipoff and Herschbach ( Mol. Phys. 2010 , 108 , 1133 ) as a "blessed-practice" for the relevance of any PES. In the present case significant differences were revealed between the two types of dressed-adiabatic PESs, eventually, indicating that the lowest, ab initio PES (due to the Born-Oppenheimer approximation) is not adequate for low energy processes.

Study of Topological Effects Concerning the Lowest A″ and the Three A′ States for the CO2+ Ion

“…A study of this kind was not too long completed for the F + H 2 system 59−61 and among other things it was verified that the common PES based on the BO approximation is most likely to be inadequate. 61 In the present article, just like in a recent study for the NH 2 , 62 these studies were extended significantly in the size of configuration space as well as by treating simultaneously the JT effect and the RT effect to show that a rigorous study of the CO 2 + system is feasible. In other words, this study guarantees that diabatic PESs on the ab-inito level can be produced for the title system.…”

“…Deriving such potentials for polyatomic systems is vital for being able to analyze processes such as chemical reactions, inelastic collisions, unimolecular dissociation, spectroscopic transitions, charge transfer, etc. A study of this kind was not too long completed for the F + H 2 system − and among other things it was verified that the common PES based on the BO approximation is most likely to be inadequate …”

“…Thus, along the interval, 0 ≤ φ < π, the relevant angle γ 12 ( r |φ, q ) is identical to the two-state ADT angle given in eq whereas for π < φ ≤ 2π, it is seen that γ 12 ( r |φ, q ) is similar but shifted by an amount χ­(φ=π). Thus, γ 12 ( r |φ, q ) becomes a step-function: where χ­( r |φ=π, q ) is (a constant) derived to be of the form ,− …”

Benchmark Quantum Mechanical Calculations of Vibrationally Resolved Cross Sections and Rate Constants on ab Initio Potential Energy Surfaces for the F + HD Reaction: Comparisons with Experiments

“…Except for this discrepancy, the general good agreement with the experimental data of the Born− Oppenheimer dynamics here presented leaves little room for nonadiabatic effects coming from the conical intersections of this system. 45,46 More relevant, from the theoretical point of view, is probably the extension of the accuracy of the Born− Oppenheimer PESs employed in the cold and ultracold regimes, where most of the PESs here employed fail to give stable ics behaviors (see comments above and in Section 2). This is particularly important now that experimental techniques 80 have appeared to be able to cover low temperatures, and likely new experiments will be soon available for this reaction and its isotopic variants.…”

“…There is also another more fundamental reason to understand how closely the new PESs reproduce the experimental observables. In fact, it is well-known , that for the F + H 2 system important Renner-Teller and Jahn–Teller effects could be operative near the conical intersections located in three different regions along the minimum energy path of the entrance channel. Spin–orbit may not therefore be the only nonadiabatic coupling affecting the reactive observables.…”