A New Approach To The Evaluation Of The S-Matrix In Atom-Diatom Quantum   Reactive Scattering Theory
  A new approach is described to the evaluation of the S-matrix in three-dimensional atom-diatom reactive quantum scattering theory. The theory is developed based on natural collision coordinates where progress along the reaction coordinate can be viewed as fulfilling the same role as time in a time-dependent formulation. By writing the full wavefunction in coupled-channel form it is proved that the 3D quantum reactive scattering problem can be treated in the same way as an inelastic single-arrangement problem. In particularly, two types of coupled-channel representations, which are reduced to two different systems of coupled first order ordinary differential equations describing the inelastic scattering, are used. The first system of coupled differential equations is constructed on a set of points (grid) of the coordinate reaction curve after solution of many 1D Schroedinger problems in the directions normal to the reaction coordinate. The second expression for inelastic scattering is found using exactly solvable nonstationary 1D Schroedinger equation (etalon equation method), which is introduced for describing the localization properties of the full wavefunction along the curve of coordinate reaction. In this case we avoid a large amount of computation involved in solving the 1D Schroedinger problem along the reaction coordinate by using a slightly difficult initial conditions for the inelastic scattering equations. In both cases by solving the system of coupled first order ordinary differential equations, the full wavefunction and all S-matrix elements are obtained simultaneously without further calculations. Our analysis shows that the methods we have developed constitute the simplest algorithms for computing the reactive scattering S-matrices. 
Ashot S. Gevorkyan, Gabriel G. Balint-Kurti and Gunnar Nyman