Mathieu LEWIN Solution of a mountain pass problem for the isomerization of a molecule with one free atom (332K, POSTCRIPT) ABSTRACT. In this paper, we continue the mathematical study of adiabatic chemical reactions, started in a previous work (Ann. Henri Poincar , 5: 477--521, 2004). We consider a molecule with one free atom, the latter having two distinct possible stable positions. We then look for a mountain pass point between these two local minima in the non-relativistic Schr dinger framework.