Large curvature tunnelling on the reaction path.

The reaction path Hamiltonian [J. Chem. Phys., 1980, 72(1), 99] has found few if any successful applications to the explicit treatment of hydrogen tunnelling dynamics in the limit of large path curvature which is typical for hydrogen exchange tunnelling. I argue hat this is due to fundamental limitations of the reaction path formulation. A two-dimensional model mimicking the intramolecular hydrogen transfer in malonaldehyde shows that commonly used approximations can produce misleading results. As a solution to the problem I propose a non-orthogonal representation of the Hamiltonian in terms of local harmonic oscillators distributed along the reaction path. In contrast to the usual reaction path Hamiltonian it has an exact variational limit for all curvatures. This distributed harmonic oscillator approach has the added advantages of stability against variations of the reaction path as well as the ability to treat bifurcating paths.