Coarse-Grained Kinetic Computations for Rare Events: Application to Micelle Formation

TL;DR

This paper presents a method to efficiently compute rare micelle formation events by deriving coarse-grained dynamics from microscopic simulations, avoiding explicit Fokker-Planck equations, and validating the approach against detailed simulations.

Contribution

The authors introduce a novel approach to extract coarse-grained free energy landscapes directly from microscopic simulations without deriving explicit Fokker-Planck equations.

Findings

Coarse-grained free energy surface matches full GCMC results.

Cluster size is a valid reaction coordinate for larger clusters.

Small clusters require alternative reaction coordinates.

Abstract

We discuss a coarse-grained approach to the computation of rare events in the context of grand canonical Monte Carlo (GCMC) simulations of self-assembly of surfactant molecules into micelles. The basic assumption is that the {\it computational} system dynamics can be decomposed into two parts -- fast (noise) and slow (reaction coordinates) dynamics, so that the system can be described by an effective, coarse grained Fokker-Planck (FP) equation. While such an assumption may be valid in many circumstances, an explicit form of FP equation is not always available. In our computations we bypass the analytic derivation of such an effective FP equation. The effective free energy gradient and the state-dependent magnitude of the random noise, which are necessary to formulate the effective Fokker-Planck equation, are obtained from ensembles of short bursts of microscopic simulations {\it with judiciously chosen initial conditions}. The reaction coordinate in our micelle formation problem is taken to be the size of a cluster of surfactant molecules. We test the validity of the effective FP description in this system and reconstruct a coarse-grained free energy surface in good agreement with full-scale GCMC simulations. We also show that, for very small clusters, the cluster size seizes to be a good reaction coordinate for a one-dimensional effective description. We discuss possible ways to improve the current model and to take higher-dimensional coarse-grained dynamics into account.