In its original form , this choice of K allows writing the absolute potential using properties of just a single electrode. Herein, it enables writing Eq. ( ) as U M sol (abs) = ∆ M S ϕ -µ M e + μR e = ∆ M S ϕ -ϕ M,R , which, in turn, allows direct control of ∆ M S ϕ without explicitly introducing the Fermi level after choosing ϕ M,R . In practice, we pick ϕ M,R to correspond to the potential zero charge of the electrode M : as shown in the supporting information Eq. S1 this choice enables controlling the applied potential by controlling the inner potential ϕ M . While this reference scale is not experimentally viable, it can be used in simulations as demonstrated very recently by the application of density-potential functional theory to electrochemical interfaces. The Supporting Information, Section S2 further shows that the inner potential scale is natural for (GCE-)DFT simulations and leads to CIP-DFT.