Atom-surface scattering accurate calculations

The development of theory for reliable calculations of chemical dynamics has two components the construction of accurate, ab initio, multidimensional potential energy surfaces (PESs) and the performance of reactive scattering calculations, either by time-independent (TI) or time-dependent (TD) methods, on these surfaces. Accurate TI quantum methods for describing atom-diatom reactions, in particular for the benchmark H + H2 reaction, have been achieved since 1975.[1,2,3] Many exact and approximate theories have been tested with the H + H2 reaction.[4,5]... 

The Landau-Zener model illustrates the important variables influencing the probability of non-adiabatic transitions, but as a ID model it is only applicable to bimolecular reaction of two atoms. For most reactions of interest it is too simple to provide accurate results. For reactions involving more than two atoms the PESs are multidimensional, as we have seen above, and the avoided crossing region on a multidimensional surface is described as a conical intersection [61]. The best method for handling this complex multidimensional reactive scattering problem is trajectory calculations. Fernandez-Ramos et al. [52] has discussed approaches to this problem as part of a recent review of bimolecular reaction rate theory. It is fortunate that the vast majority of chemical reactions occur adiabatically. It will only be necessary to delve into the theory of non-adiabatic reactions when a non-adiabatic reaction is present in a reaction model, experimental data are not available, and the reaction rate influences the overall rate appreciably. 

The reactions H + H2 and F + H2 (and their isotopic variants) have been the benchmark systems in the field of chemical reaction dynamics. For them, fully converged three-dimensional (3D) quantum scattering calculations of state-to-state differential cross sections (DCS) have been performed and accurate comparisons with very detailed experimental observables carried out [3-9]. To date, only for one other neutral three-atom system have the exact (i.e., fully converged) 3D quantum scattering calculations of state-to-state DCS on a reliable ab initio potential energy surface (PES) been carried out, namely, for the prototypical reaction Cl + H2 [10], a system chemical kineticists have been interested in since the time of Max Bodenstein (for a historical overview, see the paper by Truhlar in this volume). However, in contrast to H + H2 and F + H2, no experimental dynamical information is available on Cl + H2. Here we highlight the results of the first dynamical investigation of the Cl + H2 and Cl + D2 reactions by the crossed molecular beam... 

In order to reliably predict the rates for these processes, it is necessary to perform accurate dynamical calculations using accurate potential energy surfaces (PES). However, one is faced with difficulties realizing these ideals. Quantum mechanical scattering methods are not yet advanced to the point where 3-dimensional calculations can be performed on three and four atom systems at elevated temperatures and accurate PES information is rarely available. Fortunately it is often possible to ameliorate these difficulties and in this paper we discuss recent advances in this area. 

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