The random force in molecular dynamics with electronic friction

TL;DR

This paper demonstrates that including a random force in Langevin dynamics is crucial for accurately simulating ballistic molecular motion with electronic friction, as neglecting it leads to significant discrepancies with experimental results.

Contribution

It shows that the random force, often neglected in ballistic regimes, is essential for realistic molecular dynamics simulations involving electronic friction.

Findings

Simulations without the random force fail to match experimental data.

The random force arises from thermal electron-hole pairs.

Ballistic particles under friction require stochastic modeling for accuracy.

Abstract

The Langevin equation includes a random force to maintain equilibrium and prevent friction from bringing motion to a standstill; but for ballistic motion, the random force is often neglected. Here, we use the Langevin equation for molecular dynamics simulations of 2.76 eV H-atoms experiencing electronic friction in collisions with 300 K metals, where a random force arises from thermal electron-hole pairs. Simulations without the random force fail dramatically to reproduce experiment, although the incidence energy is much larger than $k_\text{B}T$. We analyze the Ornstein-Uhlenbeck process to show that this is a general property of ballistic particles experiencing friction under the influence of thermal fluctuations.