Both Equations 8 and 9 require the value of the surface potential χ S . However, computing this quantity is not a trivial task, even when using more advanced techniques such as AIMD. We bypass the need for χ S by noting that this term cancels out when taking the difference between the two equations. This approach is often called the Computational Standard Hydrogen Electrode (CSHE) method, where the absolute level is directly computed from thermodynamic arguments. The method tacitly assume that the calculated surface potential (χ calc S ) and the experimentally measured surface potential (χ exp S ) are equal, that For a given phase α, ϕ α is the inner (Galvanic) potential, ψ α is the outer (Volta) potential and χ α is the surface potential.