To begin, we illustrate the reaction coordinate consisting of moving just the proton (shown in cyan in Figure ) to the surface. We will later generalise this coordinate to any number of atoms. We Taylor expand q, which is the variation of the vector containing the charge transfer components along ω. In general, between two states 1 and 2, where ω = 0 at state 1 and ω = 1 at state 2, for each dimension,