Applied Potentials in Variable-Charge Reactive Force Fields for Electrochemical Systems

Abstract

An atomic description of water dynamics and electrochemical properties at electrode-electrolyte interfaces is presented using molecular dynamics with the third generation of the charge-optimized many-body (COMB3) potential framework. Externally applied potentials in electrochemical applications were simulated by offsetting electronegativity on electrode atoms. This approach is incorporated into the variable charge scheme within COMB3 and is used to investigate electrochemical systems consisting of two Cu electrodes and a water electrolyte with varying concentrations of hydroxyls (OH) and protons (H). The interactions between the electronegativity offset method and the charge equilibration method in a variable charge scheme are analyzed. In addition, a charge equilibration method for electrochemical applications is proposed, where the externally applied potentials are treated by the electronegativity offset on the electrodes thus enforcing charge neutrality on the electrolyte. This method is able to qualitatively capture the relevant electrochemistry and predict consistently correct voltages with precalibration.

Optimized utilization of COMB3 reactive potentials in LAMMPS

“…It is determined that this is an effect of using the electronegativity equilibration scheme QEq which enforces charge neutrality. 4,23 It is seen that for all the different layers, the Coulombic energy convergence occurs at the same rate, and closely reflects the spreading rate of the nanodroplet on the Cu surface.…”

“…Contrary, to prior observations, a larger time step of 0.4 fs which has previously been associated with unstable simulations can be used when a lower τdamp parameter is used. 4,7 FIG. 1.…”

“…Another input parameter that was considered to optimize COMB3 potential was the frequency of charge equilibration for the system which is an important parameter for the accurate simulation of dynamical processes. 4,7 Charge equilibration is a complex calculation, since it is an iterative process dependent on an atoms local environment, that should be performed at every time step to obtain the most realistic physical event, making it a rigorous process to implement. 4,6,23 In this work, decreasing the simulation time for Coulombic charge convergence to occur was evaluated by performing charge equilibration at different time step iterations.…”

Molecular Simulation of Electrode-Solution Interfaces

“…The expression of the energy is similar in the case of the QE/EEM method, which was used to investigate electrochemical interfaces [60][61][62][63][64][65], but the expressions of the matrix A and vector B involve terms such as electronegativities and chemical hardnesses, instead of the purely electrostatic picture described above. It is also related to the split charge equilibration approach, which includes bond-specific terms in the energy [66].…”

Molecular Simulations of Electrotunable Lubrication: Viscosity and Wall Slip in Aqueous Electrolytes

“…electronegativity, Coulomb self-energy and Coulomb shielding constant, along with EEM-related Coulomb cutoff radii, were taken from Monti et al [55]. A voltage ΔΦ between the capacitor plates is simulated by offsetting electronegativity values of the gold atoms for the EEM by ± e ΔΦ∕2 [56,57]. We note that this scheme treats the electrodes as perfectly metallic, i.e.…”