Elastic Collision Based Dynamic Partitioning Scheme for Hybrid Simulations

In hybrid simulations, such as the QM/MM approach, system is partitioned into regions that are treated at different levels of theory. The key question then becomes how to evaluate interactions between particles on opposite sides boundary. One approach place boundary in a way near both same type, thus simplifying evaluation interactions. If mobile present, solvent molecules, and allowed cross boundary, conservation energy atomic forces problematic unless computational effort increased significantly. By preventing from crossing but allowing be flexible, an accurate estimate average thermodynamic properties obtained principle illustrated by flexible inner region ensemble separator (FIRES) method [C. Rowley B. Roux, J. Chem. Theory Comput. 2012, 8, 3526]. FIRES, harmonic restraint applied Therefore, it can occur particle some extent resulting anomalies density. Here, constraint presented where instantaneously scatter This scattering-adapted FIRES (SAFIRES) implementation makes use variable-time-step propagation algorithm time step scaled automatically identify moment collision should occur. length kept constant, this propagator reduces regular Langevin dynamics algorithm, velocity Verlet for conservative if friction coefficient set zero. Correct statistics demonstrated simulations where, testing purposes, two level theory, namely, homogeneous Lennard-Jones (LJ) liquid water based TIP4P potential function. order illustrate solid–liquid interface LJ contact with surface crystal also simulated. using SAFIRES shown reproduce unconstrained reference without significant deviations density conserve when coupling heat bath turned off.