Constant charge method or constant potential method: Which is better for molecular modeling of electrical double layers?

Abstract

In molecular modeling of electrical double layers (EDLs), the constant charge method (CCM) is prized for its computational efficiency but cannot maintain electrode equipotentiality like the more resource-intensive constant potential method (CPM), potentially leading to inaccuracies. In certain scenarios, CCM can yield results identical to CPM. However, there are no clear guidelines to determine when CCM is sufficient and when CPM is required. Here, we conduct a series of molecular simulations across various electrodes and electrolytes to present a comprehensive comparison between CCM and CPM under different charging modes. Results reveal that CCM approximates CPM effectively in capturing equilibrium EDL and current-driven dynamics in open electrode systems featuring ionic liquids or regular concentration aqueous electrolytes, while CPM is indispensable in scenarios involving organic and highly concentrated aqueous electrolytes, nanoconfinement effects, and voltage-driven dynamics. This work helps to select appropriate methods for modeling EDL systems, prioritizing accuracy while considering computational efficiency.