The treatment of the electrode potential remains a central challenge in atomistic simulations of electrochemical interfaces. A finite supercell represents only a microscopic cutout of the macroscopic system and requires boundary conditions that mimic the missing environment. Conventional approaches impose either constant-charge or constant-voltage boundary conditions. Here we show that each reproduces the dynamical behaviour of the electrode potential only in one region of the supercell - either at the electrode-electrolyte interface or at the artificial boundary where the electrolyte is truncated - while the other is suppressed. Static boundary conditions are therefore intrinsically incapable of capturing the full potential dynamics. Yet electrochemical reactions are governed not only by the average potential, but by its fluctuations in space and time, which directly control local reaction pathways and barriers.
By introducing dynamic boundary conditions via the thermopotentiostat, ab initio molecular dynamics simulations are able to reproduce the correct distribution of potential dynamics throughout the microscopic simulation volume. To highlight the performance of this technique, we apply it to the 150-year-old puzzle of anomalous anodic hydrogen evolution during magnesium corrosion. Our simulations reveal a transiently stable Mg²?(OH)? ion complex that behaves effectively as a unipositive Mg species. These findings resolve a long-standing controversy: coulometric measurements consistently indicate unipositive Mg, while spectroelectrochemical techniques have never detected it. This example illustrates how dynamic boundary conditions provide access to mechanistic insights that remain hidden under static control, and underscores their essential role in enabling predictive ab initio electrochemistry.
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