Simulations of chemical reactions including explicit solvent are challenging. High computational accuracy is required to obtain reliable energetics and kinetics of the reaction. However, inclusion of many solvent molecules in such calculations is prohibitively expensive. Hybrid methods which couple different levels of theory, such as the QM/MM approach, can provide a trade-off between high accuracy at the reaction center and lower accuracy of the (still explicit) solvent environment. But, where to place the boundary between the coupled regions? Having solvent molecules on both sides of the boundary simplifies the electronic interaction parameters. If the boundary is permeable, simulations suffer from costly-to-resolve energetic discontinuities when molecules transition between regions. If the boundary is impermeable, trajectories of molecules are disturbed by redirection at the boundary. However, such restrictive hybrid methods are shown to produce correct average thermodynamic properties while remaining computationally efficient. The Flexible Inner Region Ensemble Separator (FIRES) method presented by Rowley and Roux in 2012 provides a rigorous theoretical basis for such an approach. In the original FIRES implementation, a harmonic restraining potential impede molecules in traversing the boundary. This, however, produces artifacts which are addressed by other potential-based implementations (<i>e.g.</i> BEST, BCC) using corrective approaches, thereby increasing the originally low computational effort of FIRES. Here we present the Scattering-Adapted FIRES (SAFIRES) method which avoids artifacts by resolving boundary conflicts through instantaneous elastic collisions instead of a finite range potential. SAFIRES implements a new variable-time-step MD integrator to propagate the system to the moment in time where a collision occurs. SAFIRES is shown to conserve energy in MD simulations when coupling to a heat bath is turned off and can be carried out with negligible computational effort. Benchmark studies will be presented where results obtained using SAFIRES are compared with results from unconstrained simulations and simulations using FIRES.

Scattering-adapted flexible region ensemble separator: A fast, robust, and energy-conserving boundary method for hybrid simulations

ACS Spring 2022

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