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The classical reaction theory of Kramers describes transitions over a potential barrier (activation processes) by Fokker-Planck equations or the corresponding Langevin equations. Kramers model is based on the assumption of uncorrelated stochastic forces. In this work simple but more realistic microscopic models for transitions are developed. The physical forces leading to transitions are studied by means of molecular dynamics simulations. The investigation is restricted to two-dimensional systems. In particular we study local energy excitations of soft molecules solved in liquids. This investigation makes use of an analogy to Toda lattices. For Toda systems it was shown in earlier work that soliton excitations and soliton fusion at the soft sites may lead to local energy spots. The basic configuration studied here is a soft reacting site imbedded into a thermal bath of hard molecules. In the first part we develop a microscopic model for Kramers bistable potential well and calculate the transition rates by MD-simulations. In the following parts we study models for the dissociation of molecules and for the fusion of molecules.

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