Hybrid explicit-droplet/implicit solvation model to accelerate constant-potential molecular dynamics simulations

Using hybrid solvation model has become an important way to simulate the dynamics of electrochemical solid-liquid interfaces under realistic solvation and potential. However, since it typically relies on fully covering explicit solvent layers, it suffers from high computational cost and the dissolution of solvent molecules into the implicit solvent. To address these challenges, we present a hybrid explicit-droplet/implicit solvation model implemented based on VASPsol++, which enables efficient constant-potential molecular dynamics simulations around local reactive sites. This model employs an algorithm to exclude implicit solvent within the droplet and a velocity-reflection algorithm that prevents explicit solvent molecules from dissolving into the implicit solvent. It features both radius- and density-constant implementations and integrates a continuous cavity. Validated with established water-layer models, the droplet approach reliably replicates key interfacial properties, such as electron-count fluctuations and free-energy barriers from enhanced sampling calculations, in exemplar systems including Co-N-C motifs and MoS2 edges. Notably, this model accelerates barrier-calculation speed by 2-4 times, depending on slab size and specific settings, while providing reliable results. This study offers a new tool through which simulating the electrochemical interface using constant-potential molecular dynamics is significantly accelerated and more broadly accessible.