Recent Advances for Improving the Accuracy, Transferability, and Efficiency of Reactive Force Fields

Abstract

Reactive force fields provide an affordable model for simulating chemical reactions at a fraction of the cost of quantum mechanical approaches. However classically accounting for chemical reactivity often comes at the expense of accuracy and transferability, while computational cost is still large relative to non-reactive force fields. In this Perspective we summarize recent efforts for improving the performance of reactive force fields in these three areas with a focus on the ReaxFF theoretical model. To improve accuracy we describe recent reformulations of charge equilibration schemes to overcome unphysical long-range charge transfer, new ReaxFF models that account for explicit electrons, and corrections for energy conservation issues of the ReaxFF model. To enhance transferability we also highlight new advances to include explicit treatment of electrons in the ReaxFF and hybrid non-reactive/reactive simulations that make it possible to model charge transfer, redox chemistry, and large systems such as reverse micelles within the framework of a reactive force field. To address the computational cost we review recent work in extended Lagrangian schemes and matrix preconditioners for accelerating the charge equilibration method component of ReaxFF and improvements in its software performance in LAMMPS.

Can Electric Fields Drive Chemistry for an Aqueous Microdroplet?

“…46,47,48 . In this work we have utilized a reactive force eld model of water, ReaxFF/C-GeM 49,50 , that explicitly models coarse-grained electrons 45 , and yet can handle the relatively large sub-micron droplets simulated over tens of nanoseconds that we have examined here that is not accessible to ab initio molecular dynamics. Figure 1B veri es that the Stark shift trends are very well captured by the ReaxFF/C-GeM model, which is relevant for not only the validation of the simulation model, but plays an important interpretative role in analyzing the electric eld for large water droplets in terms of electron density, protonation states, and ion effects at the air-water interface.…”

“…The cubic box was set to be (80 Å) 3 , (120 Å) 3 , (160 Å) 3 , and (200 Å) 3 for R40, R60, and R80 droplets, respectively. The systems were then transferred into a recent implementation in LAMMPS 50 , where the reactive force eld ReaxFF/CGeM model has been implemented and the MD trajectories were conducted. After 500 ps of equilibration, we collected snapshots every 1 ps across a 400 ps production run to obtain the electric eld.…”

Can electric fields drive chemistry for an aqueous microdroplet?

“…The cubic box was set to be (120 Å) 3 , (160 Å) 3 , and (200 Å) 3 for R40, R60, and R80 droplets, respectively. The systems were then transferred into a recent implementation in LAMMPS 39 , where the reactive force field ReaxFF/CGeM model has been implemented and the MD trajectories were conducted. After 500 ps of equilibration, we collected snapshots every 1 ps across a 400 ps to 1 ns production run to obtain the electric field.…”

“…In this work we have utilized a reactive force field model of water, ReaxFF/C-GeM 38 , 39 , that explicitly models coarse-grained electrons and thus the internal electronic charge distribution of the water molecule 40 , and yet can well describe the structural organization and dynamics of water for relatively large sub-micron droplets over tens to hundreds of nanoseconds, size and timescales that are not accessible with ab initio molecular dynamics (AIMD). Here we use the model to simulate the electric fields of large droplets of 80–160 Å in diameter to characterize their field strengths at the air-water surface, and to evaluate the electric fields for different charge states of the water droplets with an excess of Na + ions, Cl − ions, H 3 O + ions, OH − ions and Na + /Cl − ion mixtures.…”

Valence energy correction for electron reactive force field

“…18 The electrostatic interactions can be calculated by the electronegativity equilibration method (EEM) 37,38 when performing standard ReaxFF, however, it includes inadequacies such as long-range charge transfer, noninteger molecular charges at large separations, out-of-plane polarization, and so forth. To impede unphysical long-range charge transfer, the atom-condensed Kohn-Sham (ACKS2) 39 approach provides an extension to the EEM that improves the atom-in-molecu description e. 40 In eReaxFF, the electron and electron-hole can modify only the over-coordination, penalty, under-coordination, and lone pair energy stability, while ACKS2 describes the electrostatic interactions.…”