(316d) Accelerating Ring-Polymer Molecular Dynamics Simulation: A Parallel-Replica Dynamics Approach

Classical molecular dynamics (MD) method is a powerful tool for simulating materials on the atomic scale, which can be used to compute a wide range of equilibrium and dynamical properties, such as elastic moduli, diffusivity, free energy barrier and reaction rate. However, classical MD neglects quantum mechanical nuclear zero-point energy and the nuclear tunneling effects. For systems containing light elements at low temperatures, quantum corrections are necessary in order to obtain a quantitative correct description of the system.

While solving the full time-dependent Shrodinger equation is usually limited to small systems, there are several approximate methods that have been developed for dealing with those complex, many-body systems. Among them, the ring-polymer molecular dynamics (RPMD) method is a popular semi-classical approach to include quantum nuclear effect in a classical MD simulation. RPMD is based on the path-integral formalism of statistical mechanics, which yields real-time MD trajectories that preserve the exact quantum Boltzmann distribution. It has been demonstrated in several systems that RPMD method does very well when used to calculated thermal reaction rates.

A long-standing problem is that conventional MD is limited to relatively short simulation timescales, which leaves many important phenomena out of reach. These so-called activated processes are typically characterized by long periods of uneventful vibrational dynamics punctuated by rapid transitions. While very powerful, RPMD suffers from an even more severe timescale problem as trajectories are generated not on a single system, but on a ring polymer where each bead is a complete replica of the system.

In classical MD, accelerated molecular dynamics (AMD) methods address this challenge by concentrating on the sequence of transitions and shortening the inter-transition intervals. AMD methods require no apriori knowledge of the possible transitions, but to modify the dynamics so that transitions occur faster. In the present work, RPMD was coupled with Parallel Replica Dynamics (ParRep) method to accelerate the simulation of systems containing light atoms at low-temperatures. An example system of helium atom diffusing in the bulk Fe, W, and Fe-Cr alloy at various temperatures will be discussed.