Direct calculation of kinetics from free energy surfaces

Many physical and chemical transformations involve metastable states with lifetimes that exceed the time scale of standard molecular dynamics (MD) simulations. State of the art enhanced sampling methods, on the other hand, make the reconstruction of a free energy surface (FES) nowadays fairly routine in many cases. Sampling of reaction rates, however, remains considerably more challenging and also tends to require different sampling strategies.

In this contribution, we will discuss the possibilities of inferring accurate kinetic information (i.e., rates) for various types of transformations directly from the FES associated with the process of interest. This way, kinetic and thermodynamic information can be accessed from a single simulation at no extra cost. This greatly extends the application domain of well-established free energy methods, while rates become accessible for a wider range of processes. We will show several examples—as diverse as gas-phase chemistry, heterogeneous catalysis, and nucleation—that allow us to enumerate the surprising successes and key caveats of such a generic approach to rate calculations, and discuss what can learn from them.