Estimating time-correlation functions by sampling and unbiasing dynamically activated events

TL;DR

This paper introduces a novel method combining biased sampling based on Jacobian eigenvalues with unbiasing via MBAR to efficiently estimate time-correlation functions in many-body systems, demonstrated on vacancy migration in iron.

Contribution

It proposes a new biasing technique using the lowest Jacobian eigenvalue moduli and integrates it with MBAR for unbiasing, improving rare-event sampling without endpoint constraints.

Findings

Accurately estimates vacancy migration rate in alpha-iron.

Enhances sampling of reactive trajectories through eigenvalue-based biasing.

Recycles rejected trajectories to speed up calculations.

Abstract

Transition path sampling is a rare-event method that estimates state-to-state timecorrelation functions in many-body systems from samples of short trajectories. In this framework, it is proposed to bias the importance function using the lowest Jacobian eigenvalue moduli along the dynamical trajectory. A lowest eigenvalue modulus is related to the lowest eigenvalue of the Hessian matrix and is evaluated here using the Lanczos algorithm as in activation-relaxation techniques. This results in favoring the sampling of activated trajectories and enhancing the occurrence of the rare reactive trajectories of interest, those corresponding to transitions between locally stable states. Estimating the time-correlation functions involves unbiasing the sample of simulated trajectories which is done using the multi-state Bennett acceptance ratio (MBAR) method. To assess the performance of our procedure, we compute the time-correlation function associated with the migration of a vacancy in {\alpha}-iron. The derivative of the estimated time-correlation function yields a migration rate in agreement with the one given by transition state theory. Besides, we show that the information relative to rejected trajectories can be recycled within MBAR, resulting in a substantial speed-up. Unlike original transition path-sampling, our approach does not require computing the reversible work to confine the trajectory endpoints to a reactive state.