Unified semiclassical perturbation and infinite order sudden approximation, with application to the reaction path Hamiltonian model

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Abstract
It is shown how two popular approximate dynamical models—the semiclassical perturbation (SCP) approximation and the infinite order sudden (IOS) approximation—can be combined in a consistent way that includes the correct features of both. Application of this unified SCP–IOS approximation to the reaction path Hamiltonian model of Miller, Handy, and Adam [J. Chem. Phys. 72, 99 (1980)] leads to extremely simple, explicit formulas for the reactive S matrix, product state distrubutions, etc., which can be readily applied to polyatomic systems. Initial numerical tests on a simple model problem indicate that the model is of useful accuracy.