BioExcel Webinar #18 - Multiple timescales in atomistic simulations

Rare events are processes that occur on time-scales longer than those currently accessible by brute force simulations, i.e. at most microseconds in classical molecular dynamics and up to (one or few) hundreds of picoseconds in ab initio simulations. Interesting processes in many fields are rare events: chemical reactions, conformational changes in biomolecules, and nucleation phenomena are just a few examples. These processes are usually described as transitions among states corresponding to local minima of the free energy surface of the system (metastable states). When metastable states are separated by free energy barriers, much higher than the thermal energy of the system, the dynamics is consists in persistent motion in the basin of attraction of one of the minima and rapid, but very infrequent, transitions to the other minima. Thus, the system is characterized by disparate timescales: the timescale of atomistic dynamics (femtoseconds) and the timescale of the infrequent jumps between metastable states. In this talk, Simone will introduce a theoretical framework to describe these processes and a set of methods to investigate them. In particular, the focus will be on cases in which a limited set of order parameter is insufficient to describe the initial, final and intermediate states of the process, i.e. problems which cannot be addressed by standard techniques, such as umbrella sampling and thermodynamics integration. The use of these techniques will be illustrated with examples in several fields of condensed (soft, liquid and solid) matter, showing that they allow to treat cases requiring up to thousands of collective variables