Electrified interfaces play a central role in energy technologies, from batteries and capacitors to heterogeneous electrocatalysis. The structural complexity and the presence of the electrochemical potential present significant challenges to the atomistic understanding and modeling of these interfaces. In particular, including the electrochemical potential explicitly in the quantum mechanical simulations is equivalent to simulating systems with a surface charge (typically grand-canonical DFT). For realistic relationships between the potential and the surface charge (i.e., the capacity), the solvent and counter charge need to be treated carefully. Either solvent molecules and ions are included explicitly or implicit solvent-electrolyte descriptions are adopted. The first option is computationally too demanding to achieve a representative phase-space sampling, while the second is far from the realistic structuring of the interface. Both approaches suffer from a lack of validation against directly comparable experimental data. Furthermore, the limitations of density functional theory in terms of accuracy are critical for these metal/liquid interfaces. Nevertheless, even the imperfect models allow atomistic insights in electrocatalytic interfaces with unprecedented details. MoS2 as a catalyst for HER will be our example to illustrate the insights provided by grand-canonical DFT. We will present a detailed mechanistic study of hydrogen evolution over the single sulfur vacancy (Vs) on the MoS2 basal plane, including the potential-dependent Vomer, Tafel and Heyrovsky transition states for the various possible reaction steps.