Elastic Collision Based Dynamic Partitioning Scheme for Hybrid Simulations

TL;DR

This paper introduces SAFIRES, a novel elastic collision-based partitioning scheme for hybrid simulations that accurately separates regions with different computational methods while conserving physical properties.

Contribution

SAFIRES provides an exact, energy- and momentum-conserving boundary method for hybrid simulations, improving upon previous boundary-based approaches like FIRES, BCC, and BEST.

Findings

SAFIRES accurately reproduces reference simulation results.

The scheme conserves energy and momentum at boundaries.

It effectively models Lennard-Jones liquids and TIP4P water in hybrid setups.

Abstract

The scattering-adapted flexible inner region ensemble separator (SAFIRES) is a partitioning scheme designed to divide a simulation cell into two regions to be treated with different computational methodologies. SAFIRES prevents particles from crossing between regions and resolves boundary events through elastic collisions of the particles mediated by the boundary, conserving energy and momenta. A multiple-time-step propagation algorithm is introduced where the time step is scaled automatically to identify the moment a collision occurs. If the length of the time step is kept constant, the new propagator reduces to a regular algorithm for Langevin dynamics, and to the velocity Verlet algorithm for classical dynamics if the friction coefficient is set to zero. SAFIRES constitutes the exact limit of the premise behind boundary-based methods such as FIRES, BEST, and BCC which take advantage of the indistinguishability of molecules on opposite sides of the separator. It gives correct average ensemble statistics despite the introduction of an ensemble separator. SAFIRES is tested in simulations where the molecules on the two sides are treated in the same way, for a Lennard-Jones (LJ) liquid and a LJ liquid in contact with a surface, as well as for liquid modelling simulations using the TIP4P force field. Simulations using SAFIRES are shown to reproduce the unconstrained reference simulations without significant deviations.