Reaction swath

Figure 19(a) shows the QM simulation of the differential cross-section (DCS) in the HF + D channel, over the same extended energy range as in Fig. 5. The agreement with experiment is seen to be qualitatively reasonable. The forward-backward peaking and direct reaction swathe observed in the experiment also occur in the QM calculation, although the relative magnitudes are not consistent. Thus fully quantitative agreement between QM calculations and experiment in all of the reaction attributes must await further refinements of the PES, and/or a more rigorous treatment of the open-shell character of the F(2P) atom.90...

Let me say at the outset that there is not just one and only one useful definition of a reaction path. Rather, the concept of a reaction valley or reaction swath [11] has real utility in those simple cases where the reaction dynamics takes place in a restricted region such as the valleys of Figs. 1 and 2. Depending on how we wish to perform dynamical calculations, for example, various definitions of a reaction path might be appropriate. 

Sections 3 and 4 are concerned with the representation of the potential energy information that is needed as input for the calculations of Section 2. In particular, to carry out the calculations of Section 2 we must be able to generate the PES at any point near the reaction path for small-curvature (SC) systems and at any point ir the reaction swath, as defined above, for LC systems. 

The region of coordinate space between the LC3 path at the lowest total energy for which tunneling must be considered and the region where a quadratic expansion about the MEP is valid is included in the reaction swath clearly the swath becomes wider when lower-energy tunneling processes must be considered. 

A promising starting point in the design of polyatomic PES s is to extend in some manner the methods that have been used with success for atom-diatom PES s. This should be an especially promising approach when one only requires an accurate potential in the reaction swath for an atom-transfer reaction where at most two bond lengths differ... 

In the small-curvature tunneling approximation, k(T) requires, in addition to some of the information detailed above, the curvature components Cm(,s) of the curvature of the reaction path, where each curvature component measures the projection of the curvature vector on a particular generalized normal mode direction m. Calculation of Kl-CT(r) or kPOMT(7 requires, in addition, values of the Bom-Oppenheimer potential V in the reaction swath, typically at points where it cannot be computed from the available harmonic expansion around the MEP. 

This chapter provides an account of our recent efforts to interface dynamics calculations based on reaction-path potentials and tunneling, including tunneling through the large-curvature reaction swath, with electronic structure theory. 

For intermediate reaction-path curvature, one may use either the SCSA or LC3 approximation, but even more accurate results are obtained by a least-action (LA) method.In the LA method, the tunneling paths are linear interpolations between the MEP and the LC3 paths. Thus this method does not require knowing the potential over a wider swath than is necessary for the LC3 method. 

ABSTRACT. The calculation and characterization of molecular potential energy surfaces for polyatomic molecules poses a daunting challenge even in the Age of Supercomputers. We have written a program, STEEP, which computes reaction paths (IRCs) for chemical reactions and characterizes the reaction valley centered on the IRC. This approach requires that only a swath of the potential surface be determined, a computationally tractable problem even for many-atom systems. We report ab initio reaction paths/valleys for two abstraction reactions the OH + H2 reaction, which is a simple, direct process and the H + HCO reaction which can proceed along two distinct pathways, a direct pathway and an addition-elimination pathway. We find that the reaction path/valley method provides many insights into the detailed dynamics of chemical reactions. 

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