Water in an External Electric Field: Comparing Charge Distribution Methods Using ReaxFF Simulations

The
growing interest in the effects of external electric fields
on reactive processes requires predictive methods that can reach longer
length and time scales than quantum mechanical simulations. Recently,
many studies have included electric fields in ReaxFF, a widely used
reactive molecular dynamics method. In the case of modeling an external
electric field, the charge distribution method used in ReaxFF is critical.
The most common charge distribution method used in previous studies
of electric fields is the charge equilibration (QEq) method, which
assumes that the system is a contiguous conductor and that charge
transfer can occur across any distance. In contrast, many systems
of interest are insulators or semiconductors, and long-distance charge
transfer should not occur in response to a small difference in potential.
This study focuses on the limitations of the QEq method in the context
of water in an external electric field. We demonstrate that QEq can
predict unphysical charge distributions and exhibits properties that
do not converge as a function of system size. Furthermore, we show
that electric fields within the recently developed atom-condensed
Kohn-Sham density functional theory (DFT) approximated to the second-order
(ACKS2) approach address the major limitations of electric fields
in QEq. With ACKS2, we observe more physical charge distributions
and properties that converge as a function of system size. We do not
suggest that ACKS2 is perfect in all circumstances but rather show
specific cases where it addresses the major shortcomings of QEq in
the context of an external electric field.