Understanding atomistic and molecular aspects of chemical reactions is one of the cornerstones in chemistry and biology. Characterizing reactions in time and space is challenging due to the different length-and time-scales on which the nuclear dynamics takes place [1]. For example, typical reaction times for the Claisen rearrangement [2] in solution are on the order of seconds [3] or milliseconds (in the protein) [4]. However, the chemical step (i.e. C-C bond formation and C-O bond breaking) [5] occurs on the femtosecond time scale. In other words, during 10 9 to 10 15 vibrational periods energy is redistributed in the system until sufficient energy has accumulated along the relevant 'progression coordinate' for the reaction to occur. Because the 'chemical step' is so rapid and the system concentration at the transition state is negligible, direct experimental characterization of the transition state and the dynamics between reactant and product is extremely challenging even with current state-of-the art methods, including NMR, [6] IR, [7] or x-ray [8,9] spectroscopies.Atomistic simulations have shown to provide molecular-level insight into the energetics and dynamics of chemical reactions for systems ranging from small (triatomic) molecules to proteins in the condensed phase [1,[10][11][12][13]]. An essential requirement for a meaningful contribution of computer-based work to characterize chemical reactions is a correct description of the intermolecular interactions along the entire reaction path including the degrees of freedom orthogonal to it. This involves regions around the reactants, products and the transition state(s). Intermolecular interactions in molecular systems are often represented as a Born-Oppenheimer