Maintaining the equipartition theorem in small heterogeneous molecular dynamics ensembles

TL;DR

This paper introduces a fluctuating center of mass molecular dynamics method to restore the equipartition theorem in small heterogeneous ensembles, addressing a known violation caused by momentum conservation.

Contribution

It proposes and validates a new FCMMD approach that maintains equipartition in molecular dynamics simulations with periodic boundaries.

Findings

Restores equipartition in simulations

Validates the method through simulations

Works at equilibrium and beyond

Abstract

It has been reported recently that the equipartition theorem is violated in molecular dynamics simulations with periodic boundary condition [Shirts et al, J. Chem. Phys. 125, 164102 (2006)]. This effect is associated with the conservation of the center of mass momentum. Here, we propose a fluctuating center of mass molecular dynamics approach (FCMMD) to solve this problem. Using the analogy to a system exchanging momentum with its surroundings, we work out --and validate via simulations-- an expression for the rate at which fluctuations shall be added to the system. The restoration of equipartition within the FCMMD is then shown both at equilibrium as well as beyond equilibrium in the linear response regime.