From concepts to algorithms for the treatment of large amplitude internal motions and unimolecular reactions

The formalism of the large amplitude path Hamiltonian provides a theoretical framework for the study of dynamical problems ranging from anharmonic vibrations to unimolecular reactions. A hierarchy of models at different degrees of sophistication can be elaborated and some of them have been put into practical use through the development of the DiNa package. The simplest level requires just the characterization of all the stationary points encountered along a large amplitude path and their quadratic environments. Perturbation theory can next be used to analyze the role of anharmonicity in the vibrational modulation of physico-chemical observables for semirigid systems. An extension of the same approach to saddle points allows the computation of reliable reaction rates taking into account curvature and tunneling effects. Finally, full characterization of the harmonic valley surrounding the path allows study of vibrational modulation in flexible systems and energy flow between different degrees of freedom during chemical reactions.